I've always wondered what happens if a legitimate future hall of fame player joined a team whereby his current jersey number was already retired.

Say, if Brett Favre actually wore #10 (that was his Atlanta number wasn't it?) and it stuck throughout his career, and then joined the Vikings (who already retired #10 with Tarkenton). Surely unretiring the number would be wholly unceremonious. But then, forcing a better player to yield part of his identity, the legacy he built up? What about Brett Favre's Tenford Foundation? etc.

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Usually, they just contact the player who's number is retired and unretire it for a short amount of time... if he's still alive, of course.

Happend with Jerry Rice for instance. He asked Steve Largent to use nr. 80 for one season and got permission to do just that. It happened more often, but I can't think of any more examples right now. The players themselfs don't seem to take retired numbers as heavy as some fans do.