I read your post a few times, but it's not clear what you are dong exactly.
Is this a hierarchical design where the "channel" is a hierarchical sheet, and you want to modify one of those channels?
Are you aware of the "Replicate Layout" action plugin in KiCad?
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One thing that may work for you is t first copy some part of the schematic, and then press the right mouse button and select "Paste Special" from the popup menu, instead of a normal paste.
This "special paste" has an option to preserve the UUID's that link schematic symbols with footprints on the PCB.
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As long as the reference designators are still the same between schematic symbols and PCB footprints, you can aways use these as an alternate method during "Update PCB from Schematic [F8]".
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As a third option, you can make a copy of a part of the tracks on the PCB, delete any footprints on that part, and then snap the pads of new footprints to the endpoints of those copied tracks.
As soon new footprints are placed over some old tracks, then the tracks automatically become part of the nets for which they have enough overlap with the pads of the footprints.
For this last option, you may have to change the settings in Preferences / PCB Editor / Editing Options / Magnetic Points / Snap to tracks: Always