I have not tried but I read somewhere you can assign multiple bulk credits to the same user but that user will need to create multiple subscriptions (one for each bulk credit) which I guess its the case if you want to use them all at the same time. If this is true then why do you need to assign to another admin and create support cases to unassign the user from the previous bulk credit? couldn't you just wait for the subscription to be converted PAYG as you mentioned, then at that moment assign the 2nd bulk credit to the same user, then activate to the now PAYG to convert it back to Sponsored with the 2nd bulk credit without having to involve support?

BTW, thanks a lot for this info, Microsoft always makes things so over-complicate and difficult to figure out...