In my experience, the next tier resets and "may" improve the tier-specific rate marginally, but all of your exclusive offers will be reset--in other words gone. So, if you accrued free play or exclusive offers from gambling (the one time offers, once booked are gone), they'll disappear. So, if any of them are compelling, book them BEFORE hitting the next tier.

PS - if you're an unknown (without a history of gambling), go to hotel you like, say, Bellagio while not staying at any MLife properties, and do the following: (1) withdraw, say, $1,000 from their ATM (use a fee reimbursed ATM, of which there are too many to list); (2) make some short term $25 or $100 bets in a slot machine. If you're lucky, you can run some $$ in/out without losing or, even, winning, and they may think you're a potential high roller triggering some one time exceptional discovery offers. Comped room, free play, and $300/day food credit.