Do you grandfather in old clients when you change your rates? Why or why not?

I'm just now getting into house sitting after doing dog boarding on Rover for years, so I'm still figuring out my rates. I've only had one client so far, with one other interested in booking me the next time they leave town. I'm considering lowering my base rate but adding an additional dog rate, which would affect my one client who said they'll be booking me again. Do you give old clients your original rate or do you charge them the new, higher price? I've seen some sitters say that they grandfather in people who had lower rates, and I'm wondering if you just do this to make sure you don't lose them or if there's another reason. I'm also wondering if you only do that only for repeat clients or if it gets hard to keep track of. Thanks all!