Usurped at work by younger person

Please make sure you're in a union (apologies if I'm teaching granny to suck eggs)

Stick with it, do what needs to be done and hard as it is, swallow your pride.
As others have said, the rest of your colleagues may also be very much on edge with having a new boss with "new" ideas.
One thing I will say, is keep a record of all occasions such as this, including any photos, paperwork etc. If you record anything that involves other people, you must ask permission from them to do so.
Further along the line, if needed your evidence can be used at an industrial tribunal , should the worst come to the worst.

This happened to me years ago and fortunately I was in a position to leave them to it and take early retirement but after a few weeks I was asked to go back and help out. I didn’t go back but perhaps you could just sit back and watch what happens, they may well need your expertise but it’s up to you how you pass on your knowledge, we were all young and ambitious once.

Nononan and peasblossom are very much on my wavelengh. Plus this happened to me too. I was upset but kept my head down and got on with the work I was provided with way below my capabilities but then the favoured colleague made a lot of rash decisions and I was called back to 'assist'. As irritated as I was I just got on and found a new enjoyment in the work I was set and admit found the new comer left.

A similar thing happened to me a few years back. Do document everything, I mean everything! If you are asked to do something send an email confirming what was discussed, this will come in very handy should the very worst happen - good luck.

I feel for you. I’ve been in that situation and it was horrible. New boss, new colleague who, unlike yours, was only a couple of years younger than me. My advice is to stay strong. I let it get to me and ended up off work for 6 weeks. In my late 50s, I changed jobs and worked in a new organisation till I retired. Keep your head up, appear helpful and be kind to yourself. Lots of love. ?

I disagree. Taking personalities out of it, it's all about the actual work that has to be done. Management will be judged on efficiency rather than causing staff upsets.

This is a horrible thing to happen and unfortunately is becoming more common as people have to work longer to access their full pension than originally planned, with financial repercussions for management. It is also a strategy encouraged on management courses for the young, ambitious and thrusting, (attack more experienced members of staff) with no regard for whole staff management strategy.
You do need to cover your back so keep a journal, as unemotional as possible and in a safe place, but don't dwell on it. Make sure your work is of its usual high standard and maintain pleasant relations with the newest member, but stand your ground if necessary. Do consult your union rep and register the fact you were not given time to read a proposal concerning you before being outvoted on it, and clarify your position.
This is not about you and your personal competence, this is about two inexperienced but ambitious people seeking self-advancement at your expense. Concentrate on doing your job well and enjoying your last few years of work, and as you say, maintaining your dignity and self-respect.

The other colleagues who you regard as friends are probably watching their own back also.

Wise words, thank you so much to everyone.

I agree with peasblossom work with them and just wait until they come across a situation. One they cannot resolve but you could with your expertise. You are taking this situation to heart and stressing about about it. Your in the last couple of years of work that you obviously enjoy try to relax more.

It's interesting that even friendly work colleagues are avoiding answering your questions. Without wanting to be unkind is it possible that your boss might be doing the right thing? Ie appointing a fresh pair of eyes? While your distress is obvious and very understandable, you clearly have insight which is reflected in your post.
I think some good suggestions have been made especially about keeping a record of what's happened and also how that made you feel. Do you have supervision meetings with your boss or is there a private and formal way to raise how you feel about being so apparently sidelined?
This is an upsetting time for you. Always remember that you won't always feel like this. This too shall pass.

I'm with Nonogran and think that's an excellent idea. If you have a smart phone or even camera phone take photos or make pdf s of anything that might be said that is derogatory towards you. I don't think it is professional to not allow you time to read through the new plan and disagree that it is not a union matter. If anything you need to discuss it with a union rep and let them know how bad this has made you feel. There may be room in the new plan for you to take an active role and support the younger colleagues as you head towards retirement. Try not to feel bad and don't listen to rumours. It might in the end be a fresh way of dealing with things.

New bosses tend to do this. They want to build “their team”. It was an unpleasant way to do it and reflected his own inexperience in the management role. Younger colleagues are ambitious and “thrust” into roles they want.

If you can reframe what happened as a statement about them rather than you and your competence it might help your self esteem. What has happened is not about your personal competence, the same thing would have happened to anyone occupying your role. It’s about what they wanted, it just happened to be you that was in the way.

On a practical level you can decide to be helpful, but start stepping back, seeing it as a preparation for eventual retirement. Or if you don’t feel ready for that you could ask for a different role and a new project, as this colleague will now be dealing with your area. That will make life more difficult for them without your experience and expertise, so really it’s how far you can grit your teeth and be supportive to them or whether you’d prefer to leave them to

I really hope you can find a way to still enjoy your final years at work.

Hello LB@60
I didn't want to pass by without sending you a kind word of support.
In your shoes I think I'd start keeping a work journal. A physical notebook or something on your computer at home maybe? Without emotion describe what the situation/project is/was that you're presented with and then under a separate heading write How It Made Me Feel.
This might help to distil and calm what's going on in your head and might help as evidence should the need arise? It sounds as if there might be a bit of "constructive dismissal" creeping in here so evidence is key if you need to take it further.
I do hope you can continue with peace if mind. I am sorry it's making you upset.