Maybe I've been lucky, but I've never cut a seal to remove a screen and the ratio of
successful removal & replacement (using the original seal) to cracked screen is probably
about 100 to 1.

Slow and steady is the rule when using the blunt screwdriver to feed the seal over the
lip of the windscreen opening, then plenty of waxoyl when 'roping' the assembly back
into place.
Nylon baling twine is ideal for that purpose, although you need to form some loops on
the ends of the length of twine and also be sure to wear gloves.
I cross the twine at the middle of the lower edge of the screen, as it's fairly easy to pull
that into place first, then work upwards on each side, doing a short length at a time.

The only seal I've found which was beyond use was on a 1960s van which had spent most of its
life in southern France, where the amount of exposure to strong sunlight was probably
a lot more than we get on these islands.
Mind you, the seal on my 1971 Dyane will be replaced when that's taken apart for its
chassis change next year, but that's after 40 years of living outdoors.

ken

p.s. It's best if you can avoid doing this particular task in cold weather, as the chances
of breakage will be higher when the rubber is cold and less flexible.