copied files from SOURCE to TARGET but now want to rsync SOURCE to existing TARGET

I consolidated a lot of files+folders into one large 300 GB SOURCE folder.

Then used copy to backup the 300GB to TARGET.

Can I run rsync now from source to target ? WITHOUT it re-copying the 300GB ?

I know the more correct way is to rsync the 300GB SOURCE to an empty TARGET folder.

I first copied the 300GB because a COPY almost never fails and an rsync fails much more often.

Would is be better to delete the existing 300GB TARGET and use only rsync ?

TIA