I would not even consider upgrading in place. Set up a new 14 Distro system and migrate your settings using this script:
https://wiki.freepbx.org/display/PPS/Elastix+and+PBXinaFlash+to+FreePBX+Distro+Conversion+Tool
Then check this thread to get a list of things that require attention post migration: How to: Upgrade FreePBX13 to FreePBX14 on Hyper V - Guide
Then you need to figure out how to copy the custom dialplan stuff over, if you even can.