They can change the Immigration Rules more or less at the stroke of a pen - all it needs is for the amendment to be laid before Parliament for (I think) 28 days, and it then comes into force automatically, unless Parliament debates it and decides otherwise.
But Nationality is different - most of that is in "primary legislation" - ie the British Nationality Act 1981, which takes a lot more time and effort to change, involving months of debate in both Houses. I haven't got a crystal ball, but I can't see major changes in the residence requirements being introduced in the near (or even distant) future.
The recent increase in the length of time it takes to get ILR means that (for many people) it now takes an extra year to get to "the starting blocks" for naturalisation, and that's the sort of thing they can do by manipulating the Immigration Rules. But to actually change the residence requirements for naturalisation - or the length of time you have to have ILR - would take an Act of Parliament...
So I think you're safe - but I'm not thinking of changing my user name to Mystic Meg....
Congrats on passing the test, incidentally. It is great to see so many positive messages like that - although I hasten to add that I claim no credit whatsoever for that side of this site - I've never even been in there, let alone contributed to it! But I'd like to join in with your vote of thanks to our excellent hosts....
paul