I did an MFU less than 2 weeks before departure to SFO/LAX, and there was no problem or comment made about a 21 day limit.

I was going to do a one way MFU due to availability, but they insisted that they had to charge the full return upgrade mileage. Then they told me one way upgrades weren't allowed at all, even when it was due to no availability. To be honest, I wasn't conviced they knew 100% what the policy was as I was passed from person to person.

As it happened, they contacted inventory (or whatever the department is called) and extra award seats were relased so I got the return trip in J.

Cheers
Bob