Check out Article XXI Section D. The members may not get to vote on any other merger. SAG-AFTRA can merge with AEA or any other union (maybe IATSE) by the Delegates at a convention.

The Union may merge with or become a part of any other organization by
resolution of the National Board that is approved by either (1) sixty percent
(60%) of the members in good standing voting in a mail or electronic
ballot; or (2) sixty percent (60%) of the delegates voting at a convention. A
merger, affiliation or consolidation with another entity shall not be
considered a dissolution under this Article.
Yikes, again .  It just keep getting worse!!!!!!!!!!!
Thanks to Yosemite Stokesberry for this eye-popping piece of information.
SAG Watchdog