Spritzhenry is probably the best person to answer this. Personally I would cut back hard after flowering and replace the arch immediately. They may be knocked back if they are Group 1 plants but certainly a Group 3 will pop up again next year and I think Group 2 would as well. Over to you Spritz.

You may find you can remove the clematis from the trellis without cutting it back too much. Will just take time and careful effort.

I'd like to know which clematis they are, Reh1959. If they're group 1, then you'll lose the flowers if you cut them back now, so best either to try to untangle them and lay them flat then re-tie on to the new arch, OR wait until they've flowered and cut them back as little as possible to do the job. If groups 2 or 3, now's the time - you could cut a group 2 back by half to buds, and a group 3 to strong pairs of buds near the base.

I will try to find out what variety they are and update my post.
Thanks for the information given so far.