I wanted to shorten up my paddle and the handle had previously been glued in with 5-minute epoxy.
Someone suggested taking a grinder to it, which sounds like it could go wrong easily.
I'd read that heating up the glued area (with a hairdryer) will soften the epoxy, so thought I'd try that first.
I don't have a hairdryer, so I boiled some water in a pot, took it off the heat and dropped the handle, with shaft attached, into the pot.

After about 5 minutes I pulled firmly on each piece, using only my hands, and the shaft came off straight away.

I had contemplated getting a hot glue gun as I thought this might make adjustments easier, but this was so easy that I won't bother getting one now.
I had already cut the handle off because I knew what length I wanted, though I think this would work if you didn't want to cut the shaft first (and just stand the paddle in the pot), if the amount of shaft you want to remove is less than the paddle insert.