For many months I took note of my games with a colleague using his real name as "non-user player".
Now, he registered in BGG and I would like to change all those logs to the new user.
Is this possible?
Not that I know of.
You could however use the "SPLU", a third-party tool that is a much better play-logger to edit your previous plays.
More info here:
Taking Play Logging Improvements into Our Own Hands
Log Plays Faster with SPLU
In any case, this is not like "tagging" in Facebook. The plays won't automatically be linked to the other user; this is more as a personal preference for yourself.