When a "deal" is no deal at all...
The problem with these so-called timeshare "trade in deals" in Mexico is that, regardless of the contract content, it is often nearly impossible to actually enforce contract performance.
This "trade-in" lure is a relatively common sales ploy in Mexico; Grupo Mayan affiliates in particular have been specifically mentioned and identified as practicing the tactic.
The "trade in" offer is just a baited hook, offering the unwary prey a grossly inflated alleged "trade in value" toward a new purchase (with it's own inflated numbers). For the lucky ones, actual disposition of the "trade in" gets handed over to a third party entity via PoA, for later "dumping" on eBay, where a few bucks can then be made on "closing costs", even if the timeshare itself sells at auction for pennies (...remember, no real value was ever actually given to the "trade in" in the first place). Of course, paying still more money out of pocket to complete a alleged "transfer" certainly has a foul aroma of its' own and is most likely just an associated second tier scam. For less fortunate customers, the "trade in" is just simply ignored altogether, leaving the customer on the hook for both the old and the new timeshares.
It's sad and unfortunate, but seeking any meaningful action or resolution from what passes for a (...ahem) "legal system" in Mexico is very likely to just be an exercise in futility.