Originally Posted by dabac

The OP specifically stated that "the freewheel body is not serviceable. It doesn't have any type of retainer, 10mm bolt or anything to take it apart."

But in a second read I'm noticing the the OP is talking about both freewheel and cassette for the same hub, which is frankly an impossibility.

So, to the OP: if you have a freewheel it can easily be replaced, and 7-spd freewheels are still readily available. And if it is a freewheel then it hasn't got a cassette that can be transferred to a modern freehub body by any normal means.