The current use of an access database results in the software becoming slow and unstable once it grows to a certain size, or where it is accessed by multiple users.
Converting this to run on a SQL server database, or providing a SQL version alongside the access version, would provide the following benefits:
Improved performance
Improved stability
No longer limited to 2GB database size
Wont have to run database compact/repair/optimise
Where customers don't have a SQL licence, the database size should be small enough that they can get away with a SQL express install.