Is my previously filed TM30 still valid after changing my visa status in Thailand?

That's gonna be how ever your particular immigration office handles TM30's

Some offices want a new TM30 every time you re-enter the country even if you're returning to the residence where you have one filed, other ones say as long as you're returning to your primary address (the address you used for your extension and 90 day reporting) you don't need to file

TM30's are not valid any length of time, one of the fields on the online and I think on the form is length of stay but it doesn't count for anything

I believe seeing as you aren't on that extension anymore, and are now on 30 day visa exempt entry that you most likely will have to file a new TM30 again

BUT

as I said that is an immigration office specific thing so slog on over to your office and ask them what their policy is 🙁

OR

try to get the extension with the TM30 you have and if it won't fly they'll let you know

It totally depends on the local immigration office if you need an updated TM30 with the latest entry date (or after that) to Thailand even in the same address as before or not.

The best is to ask your local immigration office for their policy. If you want to take just the safe side, submit a new TM30.