You will keep your seats unless you request a change. They will only make the change if seats like you requested are available (certain section, certain side, etc).
From my knowledge that is correct, the seats do not cost more, just the donation. If you donate $500 you will then be eligible for parking in the $500 level lots plus $50 for the actual parking. If someone else has better info please correct me.
Well, some seats are cheaper. If you are an NCC member and between 20-29, you get discounted tickets. If you are staff/faculty, you can get discounted tickets. If you wanted to try for the south endzone bleachers, those are also cheaper but I think they've been sold out every year they've offered them. I think all the details are on Cyclones.com or the NCC website.