As above, You may retain them at existing heights and add extra ones at that height to match, if that is your desire.
Conversely, if you find the new part M compliant heights better, then now is the time to decide that.
What you may *not* do, is make it any "worse" in terms of part M compliance. For example the requirement for new builds is that all sockets and switches are between 450 and 1200 mm, so 450 to the bottom of the sockets. If your sockets were 300mm to the bottom, then while you could keep that height, you could not move them further down to 200mm. Officially of course... in the real world no one is going to measure the height of your sockets and then try and investigate if they were at any other height previously!
Make your mind up where you want them, just make sure its right, because you're more a less stuck with it!