For removal and release, do it with the belt still mounted so if you lose your grip on the shaft then the roller and spring can't fly off, also the roller won't turn if you want to try getting a grip over the whole thing. I variously use jumbo slip-joint pliers if I can keep an appropriate grip with the jaws semi-parallel. I've also used a carpenter's F-clamp with plastic padded jaws over the belted roller or a large C-clamp (6"), which is a little more fiddly to keep on. To release and put the new one on then I generally do the retraction in a bench vise rather than on the car. You can also try folding a loose belt over the roller and held pinched back on the shaft to keep it from flying apart -careful of fingers or having it aimed at anything important.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now