DC & Mac-
I had a similar worry. Last Summer I undertook a major rehab project. In advance, I bought a 10-inch Sleep Number mattress, then realized it might not fit. I called Select Comfort and explained it might be a couple months before I could determine if it fit under the "lip." The nice lady said "No problem: You have 100 nights to try the Sleep Number mattress." (Terms and conditions are here
Turns out the 10-inch fit under the "lip." I was able to test with about 20 days to spare. The alternative would have been the 8-inch Sleep Number mattress.
We bought directly from Select Comfort. For expensive items that may be subject to returns, I find buying direct can reduce hassles.