I've got a few Aeropack retainers that I've wanted to re-use and finally found the best way to remove them from old rockets.

I tried a few things, but the best was a slow even heating with a propane torch.

The first step is to cut the retainer off with about a quarter inch of motor tube remaining. Mine were all on phenolic motor tubes, but I bet glass would work the same way. I was worried about warping the thin aluminum, but a careful heating seemed to be just fine. On the first one, I overheated it a bit and the anodizing turned a bit brown as you can see from this picture. The next ones came out just fine and I was careful to heat only through the motor tube and not directly on the aluminum. I turned them slowly and consistently to avoid over heating on one area. For best results stop heating as soon as you see the motor tube pull away from the aluminum - it basically shrinks away from the retainer as it's heated.