Yes the crimp rings are a pain in the arse.
This is the way I do it:
1. Bore a large hole in a thick block of wood (2 inch), the same diameter as the top cover and thick enough to allow the top cover to go into.
2. Remove the Phillips screw on the underside of the clock and remove the lower cover.
(This cover is not held by the cimp ring!)
3. Apply protective tape to the outermost section of the crimp ring.
4. Invert the clock assembly into the wooden block and fit a large hose (jubilee) clip around the outer section of the crimp ring.
(This will ensure the crimp ring retains its shape whilst being very carefully prised up.)
5. Using a suitable screwdriver, patiently and carefully prise the LOWER side of the crimp ring upwards. Working a little bit at a time is the best practice here and don't try to prise the ring all the way up in one go.
6. Once the crimp ring has been "folded" up enough, the crimp ring and top cover can be removed, but you'll have to lift the inverted clock assembly out of the block and release the tension on the hose clip to see if it's been released enough. If not, repeat the above steps.
When reassembling the clocks, fold the crimp ring down a little at a time. I usually start at 10mm intrvals for one pass, then down to 5mm for the next pass, then down to 1mm to finally fold the ring down.
The ring can then be painted and and when the bottom cover is refitted, it hides most of the marks made by the folding.
Hope this is to some use to some of you.
I can post pics if you like.
Regards, Will