Ok, I was bold enough to try it. :) Windows 8.1 with Bitlocker system drive full encryption, using TPM 1.2 and PIN. I got the free automatic Windows 10 download upgrade, and started it without deactivating Bitlocker.
The update process worked like a charm. There have been three automatic reboots by my count, all with disabled Bitlocker because obviously the update procedure deactivated it on its own. Only after the next boot which has been initiated by myself, I had to enter my PIN (which has been accepted).
I guess this can only work, however, if the update procedure is started from within the running 'old' Windows. If you happen to boot a Windows 10 from its installation media instead, the update process will have a hard time accessing the system drive at all. So I guess this is the single situation where the advice applies that Bitlocker should be deactivated manually.
Please note that all of this is only my guessing, and the result of one single experiment. Before I applied the update, I made sure that I knew the location outside of my machine where the Bitlocker Recovery Key resides. And I recommend everyone to do the same. :)