How remove large sheet cake from pan to racks without breaking?

If you haven't made it yet, I would try lining the pan with parchment paper, letting the parchment overhang 2 sides so you can lift it out.

I take the rack an turn it upside down on the pan. Then flip it over and the pan pulls right off. If you think it may stick, running a small paring knife around the edges will loosen it.