You might find more information in the Mexican timesharing forum. Bottom line is that you really don’t have a contract with TRM that obligates you to pay them and give them your old TS. It also doesn’t obligate them to transfer your old TS from you. I’ve known folks that did the same thing ten years ago and still have their old TS.
When I was in a similar situation, I contacted Vidanta and told them I wanted to keep my old TS for sentimental reasons. I still use it. In your situation, it sounds like it’s a Mexican RTU rather than a deeded TS anyway, which TRM would be unlikely to be able to resell. If you want to keep it, you could get Vidanta to void the trade in; if you don’t want to keep it you are better off dealing with it yourself and having Vidanta void the trade in rather than paying TRM to pretend to deal with it and leave you with the MF obligation as well as their fees.
I have heard of folks getting Vidanta to lean on TRM enough to transfer the old TS, but nothing first hand, so I wouldn’t count on it. This is a type of upfront fee TS exit deal that is combined with a sales tactic for Vidanta that likely didn’t change the price you would have needed to pay in the first place.