I would generally not expect an op-up for a revenue customer to be taken away once given. They should not give the new boarding pass in business to the upgraded passenger if there is a chance it might need to be taken back.

It is pretty inexcusable for a Platinum member to be stuck in the last row of economy through no fault of their own. At the very minimum, the OP should have been given his original seat back, but it was probably too much hassle for the gate staff to deal with moving another person who will also be complaining.

All in all, sounds like a completely avoidable mishandling of one of MH's best customers.