I have 7day Marriott award which I plan to use in Seoul, is it possible to have a cash upgrade at check-in to bigger room? No status with Marriott, but just interested, how do they treat Golds and Platinums on award stays?

Anything is possible, but why leave it to chance.

The first thing I'd do is go to marriott.com and look to see if when booking an award stay if they offer any points and/or cash upgrades. If they do then there's the answer to your question, along with the associated points/cash it will cost you for a particular room. If no upgrade award options are listed then I'd call the hotel directly and make the inquiry.

You mention cash upgrade at check-in, but I'd find out if they're available and book one before I travel if it's something I really wanted. A hotel can't offer you at check-in what it no longer has.

As for how Marriott treats golds and platinum on award stays...a search of the forum will reveal all kinds of answers, but personally I've always been treated very well on award stays and can not think of one instance when I wasn't upgraded to a nicer room or suite.

I have found that Marriotts in Asia tend to be much more likely to offer complementary room upgrades to elites than NA hotels. But giving the hotel a call or an email beforehand isn't a bad idea. You can at least let them know you are coming and they might be willing to preblock you into a nicer room.

As for the particular hotel in Seoul you are staying at, you might do a search here for past threads/posts. I did a quick search of "seoul" and found quite a few hits. So there is probably some info on past experiences already posted here.

As a plat, you usually get some kind of upgrade. While on a award stay, I find that if you mention that you are on vacation, you get a upgrade at least to a nice suite.