here is my opinion and suggestions.
Remember, all you are buying is a prepaid vacation. Whether it is worth it to move from a MP to an GM depends on how much better you like one over the other for repeat visits. For me, it was definitely worth it.
Having said that, it comes down to price and other amenities. if you accept ebay as the measure, the last two Grand mayan's Master suites sold for $6415 with a VF week, and for $5550 without a VF week. These prices include the transfer fees. I have also seen them go for somewhat less. Therefore, you could keep your MP, and buy resale for about that. That should be your baseline in negotiating.
Realistically, what they will try to upgrade you into will probably be the Bliss which is the next generation units up from the MP. The Grand Bliss is the next generation of the GM. None of these units exist yet, so you are prebuying with higher maintenance fees. They are offering and negotiating both price and amenities for upgrading - such as free golf, free spa classes, no MF unless you use, and senior fee reductions after age 75, and other things depending on your negotiating prowess.
IMHO, you will not be able to negotiate an upgrade to the GM cheaper than you can buy one for resale. However, I do think they are hungry for new cash, therefore, if you are willing to work at it, I think you can negotiate a pretty good deal into the Grand Bliss (not the Bliss) Some of the amenities are pretty good, particularly if you are a golfer (as I am). But remember, always put a vacation cost value on each amenity - what would you be willing to pay on vacation for that feature; remember you are paying for it with higher maintenance fees.
If you decide to try: Here are some other tips from my experience.
1: the threat of rescinding is your friend. Make sure they know you know that you can rescind within 5 days. For 5 days, you are in control. I rescinded twice during negotiations. For this reason, if you are serious, schedule your appointment early in the week.
2: Make sure any agreement is in the contract signed by an authorized agent (not the sales person).
3: Consider the HSI vacation club breakaway weeks as zero value. They will try to use that as a negotiating ploy.
4: Consider any potential rental income as Zero value.
I believe that most people here will advise you to avoid the sales people altogether and just buy a GM on Ebay. That is probably the best recourse, unless you like to negotiate, and are willing to work hard enough to get to the people that can. The trick is knowing the real value and what it is worth to you.
Sorry for the long post. Good luck. I will be at RM in early February; I love the resorts.
Mike