I've actually done this more than once, but every time I do it, i think to myself "this can't be right." I've done it across OS upgrades, and I've done it when my phone had to be replaced because of damage.
I want to make my new iPhone (which has on it all the pictures from the previous phone, since the upgrade process duplicated my phone's filesystem) point to the existing camera backup folder.
What I end up doing:
Turn on camera backup.
Get the URL, which now has a different name for the phone (e.g., iPhone XS instead of iPhone 7)
Modify the save folder name to point at the older backup destination (e.g. change it from "iPhone XS Camera backup" to "iPhone 7 Camera Backup.")
which now generates a duplicate entry in Sync on my desktop that doesn't sync
while the phone now syncs to the old folder entry
Delete this new meaningless duplicate entry, since the existing entry is the one that starts syncing
Sit and wait while it resyncs every photo over the old ones anyway even though they are the same.
If I don't modify the destination folder, I am creating a whole new archive. Having done this before, I have no fewer than seven entire duplicates of my photos in old sync folders, hence why I want to point to an existing one and hopefully not resync everything.
Is there a better way to do this?