With wooden grips and plugs in carbon or FG shafts, sometimes I have just sawed the t-grip and stem down until I was left with a tenon, and then I made the new grip, including a mortise to fit the tenon.
If the original paddle has a plastic grip and plug, like a Norse, it may be necessary to extract the plug and then put in a piece of dowel. Being superstitious, I slightly taper the downside end of the dowel to avoid a sharp strain transition line. I have not ever tried to remove a plug by heating.
On a wood shaft paddle, you want to do like in my first paragraph, getting the original grip off (usually having to sacrifice it) while retaining a usable tenon.
On two occasions I had cut paddles too short (trying experimental grips that didn't work) and faced a need to lengthen each paddle again. In the more interesting case, I had to add 2 1/2 inches between the top of the carbon shaft and the bottom of the t-grip. I carefully drilled two holes in the top of the wooden plug in the paddle shaft, and then glued in two 1/4" hardwood dowels. While the epoxy was setting, I fudged these so they were aligned as if part of a tenon. Then I epoxied on the t-grip, a fairly large and asymetrical grip made of elm, again getting the alignment I needed before the epoxy set.
Obviously the two dowels, by themselves, would not have been strong enough to be part of the top of the shaft. But I mixed up a thick batch of epoxy and microballoons, throwing in some elm sawdust for color, and slopped it between and around the dowels, adding some Kevlar "tape" around the transition zones. (I clipped the "tape" from the border of Kevlar cloth.) I don't recall what proportion of food wrap and subsequent sanding was done to get a smooth final surface.
So, if you should shorten your shaft too much by heating or twisting it, there is still hope!
