This sounds legit but also possibly the view of this varies from state to state.
In TX, if a VIN is altered, removed, etc. the state can inspect the vehicle and they will assign a new VIN to it - with a metal foil type sticker applied elsewhere. I've had a bike that was an AZ state-assigned VIN, too.
Or something a bit more extreme than a simple cab swap, a friend that built an extended cab 3500HD wanted to retain his original 3500HD VIN for multiple reasons - not just the clean title, but also registration and commercial usage concerns since the GVWR is indicated in the VIN (4th digit) and the HD's only came single cab for the most part, with a few crew cabs here and there. He took a 3 door extended cab 1500 and chopped it at the A-pillars and floor, grafted it to the cowl of his HD. (
@skylark I know you've seen this truck, just posting it for OP and others really)
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That's some determination! Also building a truck that GM never offered.
Richard