Yeah no idea about the Pixel 6a or how difficult it is to swap batteries, but 400 cycles is at least a couple of years, right? If the swap isn't a big hassle, just do it again. Or for that matter if the old battery is still good after 400 cycles and the phone wants a replacement, just take the old one out and put it in again.
It doesn't look too bad:
https://www.ifixit.com/Guide/Google+Pixel+6a+Battery+Replacement/152516