Not sure you could just jump from one plan to another mid flight?
Well, you won't get guidance between clearing the old FP and activating the new FP. The only way to maintain continuous NAV-mode guidance is to add/delete WPs one at a time, manually.
So you would fly on in HDG mode for that bit, and then (if the GPS makes it necessary; I don't fly with a Garmin myself) set a DCT to the first WP of the new FP.
I do this frequently because it often happens that ATC gives me a DCT to a WP which is
not on the filed route i.e. to a WP which is not among those in the GPS FP. Theoretically one should question that, because the allocation of a DCT to a WP off one's filed route raises the issue of what to do in a lost comms situation, but in reality one just goes along with ATC because they do it all the time (in the UK). The only practical issue is that one immediately loses one's LFOB (landing fuel on board) data, and the ETA/ETE etc, because the GPS no longer sees the original FP. So, having loaded that DCT WP ASAP (to satisfy ATC) I then add it to the original FP, and as soon as that happens, the GPS picks things up from there. I hope this is as clear as mud
