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Actually no, the commission of a felony does not cancel your retirement, so you have the incorrect understanding of that rule. For your military retirement to be cancelled, you'd have to be called back into the service and convicted by court martial, something that I understand can be done legally, but I have never actually seen it done. In fact, I don't even know another attorney/JAG that has seen it done. It's that rare.
As for whether or not impersonating a Colonel is a crime, there are certain Stolen Valor laws that exist for that sort of thing, but it's a matter of finding a prosecutor willing to actually do anything about it.