I can't even think of how many times I've been disappointed by a sequel due to the developer mixing it up, but then you have Mass Effect 1 to Mass Effect 2 deals. I didn't even know anything was wrong with ME1, and ME2 is miles better and puts memories of the original to shame. Same for Assassin's Creed 1 and 2.
But then you have games like Final Fantasy that were fun in the beginning but... Seriously? They haven't changed at all, in fact, they've gotten worse.
And on the other side of THAT spectrum are games like Pokemon or Battlefield that, at least to me, remain fairly fun with minimal changes.
So, experimentation is worth it. But if your changes fail, don't be afraid to revert to the original strategy... *coughSEGAcoughFIXcoughSONICcough*