Room only booking- changing to more expensive room?

I have a room only booking (paid the deposit only) and am considering changing to a slightly more expensive room. When I check out, does it just automatically charge me the difference (about $30/night, so $30 for the deposit)? I can’t get to the last screen without actually changing the booking.

Yes, it’ll tell you the difference and you’ll need to pay that as part of the checkout process. The change to your new room will be confirmed after you’ve paid that extra $30.