I don’t want to be alarmist but from reading your post and some of the replies, there are undertones of constructive dismissal and even redundancy here. Constructive dismissal is basically life being made so difficult for you, that you quit - it sounds as though there quite an element of this already if it’s making you feel so awful. For redundancy the employer has to prove that the job role of the person being made redundant has disappeared - it is the job, not the employee that is redundant. The only exception to this is ‘bumping’ where a person who has lost their own role within the company, takes over the role of another employee, thus in turn, making them redundant - this is very difficult for an employer to prove as legitimate if taken to unfair dismissal hearing.
If, as I think you suspect, the employer wants this person to take over your role fully, then they either need to find you an alternative position or initiate disciplinary proceedings if there is a perception that you are in any way deficient in the role. A ‘fresh pair of eyes’ and ‘old ideas’ have been mooted, so you need to consider whether this is a possibility. If so, remember that the employer needs to initiate proper disciplinary proceedings and firstly bring any deficient aspect of your work to your attention, starting with a verbal warning. This must involve a clear explanation of what the problem is and an agreed plan on the areas and timescales of expected improvement. If the employer believes there hasn’t been sufficient improvement then at least one written warning should follow, giving a clear explanation of where the problem lies and a timescale for improvement - ACAS advise two written warnings, with a final one being issued giving grounds for dismissal.
You really need to get your union involved now if you think that any of the above is a possibility - and even if you think not, start with a chat to your union rep to advise what’s happening. If things do then get more difficult you’ll have a record of that contact and you can go from there. In the meantime, the advice from PPs is good - keep a diary of dates and times of relevant events so that you have good evidence to support any dispute.