Security/verifiability of paper voting records

[CantGetFooledAgain]

I've become very concerned that the paper records, where they exist (i.e. everywhere except where paperless touchscreens were used) may themselves be vulnerable.

How hard would it be, after changing the election results in a GEMS system, to create a second paper trail that verifies those results? This second, counterfeit paper trail would be substituted for the real one and be used when a recount was requested.

What measures are in place to ensure that this kind of fraud does not occur, or if it does, that it can be uncovered through forensic means?