Just a personal finding, perhaps interesting for other regulars dealing with the FAIRPHONE 
Today I tried to repeat the From A13-official to A15-community in one go exercise that had been successful back in September 2025. This time, the FP3 (a different one) was (and remains) on 3.7.2-official and I deliberately locked the bootloader before the attempt. I tried Apply Update in the /e/ recovery as back then, this time to 3.7.3-A15-community. It simply did not work anymore, despite various changes of parameters before several repeated attempts.
… regardless of bootloader status (i did unlock it later on for another unsuccessful attempt), regardless of USB debugging and/or OEM unlocking being activated or not in developer options. I also re-downloaded the zip file, it returned the same error message. The error message is 100 % identical to back then – I just don’t get the choice offered anymore to go through with the installation in defiance of the signature error. Perhaps I was just lucky to slip through a code loophole back then, and it might have been closed in the meantime.