But why is that a problem?  You can have your front end display the measurements in any unit you like, so why does it matter what units they're stored in?  If you want to do this as a one-off, to convert your whole database to imperial, do something like this for each table:
UPDATE MyTable
SET
    MillimetreColumn1 = MillimetreColumn1 / 25.4
,   MillimetreColumn2 = MillimetreColumn2 / 25.4
,   ...

Before you do this, make sure that the data types for your columns are appropriate for your new units.  If your mm measurements are stored as int, for example, that's not likely to give sufficient precision for measurements in inches.  That's another reason why I recommend you not to do this.

John