Obviously advise them to do a backup before attempting the upgrade. You should probably write an exe to do the update, something to be run once at each site.

In the exe I would create some directories to 1. backup existsing data 2. hold the "new" dbc's and tables til they are ready. Then you program goes through and gets the data from the the old datatbase, via APPEND FROM perhaps, or maybe INSERT INTO. That part shouldn't be too complex, assuming the new tables are radically different. The main thing is to insure you code backs up their existsing data if they have sufficient diskspace. You can tell them to do a backup, but if at all possible be safe in case of problems.