And from waay out in left field…
You could suggest setting up a new company and transferring only that data that is needed. (Keep the old company).
You would enter the opening entries just as though changing from SAP to Navision.
The biggest benefit is that you will have all the data if needed in the old company without the problems of wading through masses of very old data.
It is also an opportunity to get rid of old customer and vendor accounts, (things do get cluttered over time).
Mull it over, you may be surprised as the positive response from the users.
If the problem really is the db size (for backups etc) then archive the old company off to a spare location outside the backup set.
You could also keep the G/L entries for several years without all the sub-ledger entries.
More disc space is not always the answer, you may need to dig deeper to find the real concerns.