dcchi

I'm guessing the answer to this is fairly simple and straightforward, but having never done this myself, I want to make sure I do it right:

I have an upcoming 7-night reservation at one of the AZ resorts at a nice gov rate (for which I am eligible as a fed gov employee, if that matters, though I don't it matters for my question here). Because I couldn't find good priced airfare for the exact days of my trip, though I plan to still arrive on the originally scheduled date, I will actually be leaving AZ a couple of days after the 7 nights (for a 9-night trip in total). This got me thinking that maybe I'd break up the trip into a 5-night stay at the resort, and a 4-night stay elsewhere (maybe seeing some sights elsewhere in the southwest, for example).

Thus, I would probably like to actually now stay 5 nights (rather than 7). However, the gov rate is no longer available for just the first 5 nights. If this matters, the reservation's cancellation policy is the Marriott standard 1 night prior.

Should I:
-check in to the hotel and simply tell them that my plans have changed and I will be leaving 2 nights earlier than originally planned? (I assume this is okay, as I'm sure for many business stays, people's plans change all the time, and as long as you have enough nights' notice, it should be fine. Is that correct?) This is probably my preferred route as long as I won't be dinged for it in some way.
-call them now and have them shorten the reservation. (Will they do that, esp. since the gov rate isn't available otherwise for those dates? If there's any chance at all that it would mess up my reservation, I'd rather not do this since my airfare is all paid up and set.)
-other options?

Thanks. I realize this is probably a simple question for others, but like I said, I haven't dealt with this myself and would love some advice.

Thanks.