I generally agree with what Set said.

I would recommend you propose a new scheme that solves the biggest
problems of the old scheme.

For example, copy the old records to a different table. That removes the
obstruction to unique constraints. It also keeps the primary table size
down.

Also, add a check to the trigger so it only performs the copy under
"normal" conditions. For example, you might perform all your importing
using a login that employs the IMPORTER role. Then the trigger can check
whether the current connection is using the IMPORTER role and not
perform the copying if it is.