Early return costs?

I currently have a 36k mile/3 year lease that is fully paid for upfront. At the rate I'm going, I'm going to hit my 36k mile limit in 2 years. A quick Google search seems to indicate that there will be early termination fees/costs if I return my car early.

Question is - does this only apply to monthly payment type of leases? I'm not sure if early return fees would apply to my situation; surely dealers would make more money selling a 2 year old car with 36k miles on it as opposed to a 3 year old car with 36k miles a year in the future? Anyone has any knowledge on this? I don't expect to get any of my prepaid amounts back of course, just wondering if I have to hand over extra fees in addition to what I've already paid (plus the regular disposition fee).

Reason for wanting to turn the car in upon reaching mileage cap: ongoing insurance costs even if I don't drive it.

And before anyone starts on the upfront payment being a bad idea... that's a discussion for another time.