I have not been to the Grand Mayans since their completion, but I can tell you that regular Mayan Palace ain't exactly chopped liver! Even Sea Gardens would look great to you if you had not seen the other Mayans!
Nevertheless, if a Grand Mayan owner deposits in RCI, the unit deposited should be a Grand Mayan. The owners had to pay the big bucks to buy or upgrade and they (and the exchangers) should get what they expected unless disaster removed the property from useful service. Someone suggested that guests shifted from Grand to MP should at least be given access to GM ammenities. That sounds like the minimum of fairness and really costs Mayan nothing but a different color bracelet. Could RCI negotiate that?