my company was just acquired. we have been managing accounting periods on calendar months, but our parent company closes on the last saturday of every month. i am thinking i will just modify our accounting periods to match theirs for the remainder of the year. so starting july shift to close 7/26. Is there any reason this is a bad idea that i'm not thinking about?

I don't think that should be too risky, but in my experience, it can be kind of painful to change dates on periods. It has to be done in a very particular way or the system yells at you and won't save the changes. I'd recommend doing it first in Sandbox anyway, but if you run into any issues like that, I'd recommend using the Multi-Calendar feature instead if you have it. Build a new calendar the way you want it and then switch your sub to use the new calendar. Again, test in Sandbox.

okay perfect! i did set up the second half of the year in sandbox and tried to see if any weird accounting stuff would happen and it all seems to be okay, just wanted to get a second opinion before trying anything in prod. thank you

Have the month through the date of the purchase, and then have another month from the next day on. Multi-calendar will be helpful indeed, just like @RJMNS said.

The real pain point is not the period transition itself, but ~1 year later whenever everyone is doing YoY reporting and has forgotten that this change even happened and everyone comes telling you that their YoY reporting is wrong/messed up. You'll want to test it and message how things will show in reports for that ahead of time and do it via email, so that you can reference it for CYA purposes.