You could put this code in the form which runs the database. Change the filename and fieldname.

USE filename
COPY STRUCTURE TO tempx extended
USE tempx
LOCATE FOR field_name=upper(fieldname)
IF field_dec = 0
	USE filename EXCLUSIVE
	ALTER TABLEfilename alter COLUMN fieldname n(10,2)
ENDIF
CLOSE data
erase("tempx.dbf")

Hope this helps.