One of the reasons to use the stand alone fusion rather than the built in fusion…things like this are way easier.
There may be a way of decomposing the clip, swapping out the source, and then jumping out to the main timeline again….i honestly don’t use fusion in Resolve too much…
But this should work.
I would suggest you put your new clip at the end of the timeline.
Go to your original fusion tree and copy everything.
turn your new clip into a fusion clip, and paste everything.
now do your connection change.
Then put that fusion clip in the proper place in the timeline.
The fusion clip is registering that original clip on the timeline as the master of the domain, but it will easily let you copy complex trees from other comps into that domain.
Glenn