You shouldn't have any problem going from v12 to v14.
Actually, you can. If you review the Report an Issue section of the forum, you'll find that there are a number of problems that have been reported to FileMaker and that are awaiting resolution.
The most serious one to date is a "jumping script line" issue that can literally scramble your script by putting a script line in the wrong spot when using the new "scripts workspace" to edit a script.
These issues can also be researched via the Known Bugs List database.