I also got TG'ed to 773 on 461 last month. As did those flying the day before. And those the day before that.
What a kerfuffle it created on board...I have never seen so much seat shifting once on board in the full cabin. My partner and I, on the same PNR were literally thrown at the seat map...with me being in 15K and them in the hideous 12E. We got to the airport early and managed to get 16AB instead. There were a lot of unhappy pax and frazzled ground staff on board trying to make it work.
I don't know where all the passengers intended for the J cabin in the 77W went because I think it was showing pretty much full up until the aircraft swap (which happened about 2 days before). Unless they undersold with the possibility of aircraft swap.
Not sure how common the TG'ing is - feel quite unlucky as we got the last day in a 3 day block of swaps and as far as I can see from flightradar it was been operated by 77W every time since.