My present bed has a 10 year guarantee, so obviously it doesn't want me to change it before then!
Seriously though, you really only change a bed if you feel you are not sleeping right and then, as fizzers says, you can simply buy a new mattress.
The bed shops will advise against putting a new mattress on an old base but that's only because they want to whip another $800 from your pocket.
I did once ask the bed manufacturer (my son at 6ft 7 needs to have his beds specially made) if there was any posture or health reason for changing the base as well and he admitted that there wasn't if you have a good quality base to begin with.
Years ago, I changed from a medium to firm mattress when I felt the bed was too soft for my old bones.
Previous to that, I downsized from a Queen to a King Single but only because I wanted more room in my bedroom.
So I suppose it all depends on your reasons - and the quality of your bed Peonyrose - what a pretty name