Controlled aggression is the key . . . . . and a bit of patience. The ducting is made of ‘tough stuff’ so can take a fair bit of ‘abuse’. I find it’s not so much the ducting that can be damaged, more the surrounding cable looms, switches and connectors so some ‘care’ is also required.
PS: I note RAVE says some ‘helpful’ things like ‘remove ducting’ and ‘replace ducting’ or words to that effect, as though it is just a straightforward simple task!!!
PPS: I also have a reel of ‘bog standard’ window/door self adhesive seal that I use instead of trying to track down pukka replacement seals. It’s pretty narrow so three layers side by side usually fits in the female part of the ducting - make sure the ducting surface is stringently clean before application and it sticks pretty good.