It makes little sense to clear the list significantly before boarding begins and clearing misconnects really should not occur before T-15 (domestic). But, I'm not aware of a policy.
Whether your UG clears at T-35 or T-45 does not affect much if anything for the upgraded passenger. But, disappointing a last minute change or SDC for a paid F passenger who wants to make a change hurts the brand. Playing games with likely misconnects who make it to the gate is also a problem for those misconnects and, in the case of inbound connections from the EU, creates all kinds of compensation and duty of care issues.