We have had success doing exactly what you are attempting to do. We collapsed a Plus GL with 30,000 accounts down to 1200 accounts.

You want to consider Account Code Change. An add-on product SRP $295 CAD. It would install through Plus WSM, and allows you to wildcard accounts and/or departments into say a closed out account, and then simply delete the account if no activity exists. It creates a batch of entries to be changed and updates all connected modules on the fly when you post the transactions.

If there is activity then you are still left with just one account.

Regards,
Brian Stief, CA
Stief Group