This has happened to me twice, both with different consequences. The first time a CO agent working for AA at Midway upgraded me on a Fokker to LGA. When I found I had stickers taken out of my account, I called AA and they reinstated the stickers and gave me an extra one for my trouble.

The second time the agent at ORD offered me an upgrade to LGA and I refused. She then gave me the upgrade anyway and did not take stickers out of my account.