I looked at the specs, it would not be an upgrade, you would be going backwards.

They are pretty much the same camera, but the T31 is 18 mpix (50D is 15 mpix), and the T3i takes video (50D does not). The 50D was a state of the art camera when it came out, but now they are building better cameras (such as yours) for a lot less money.

I would use the $500 to buy a nice lens instead of buying the 50D. The 18-200 lens is probably just a kit lens (unless it is the high dollar one which runs nearly $1000 used). We need to know which lens you are looking at. The 18-200 runs from $120 -$1000 used, depends on the type.