I just realized the Phatty has a firmware upgrade available (OS 3.1)
What does it take to upgrade my Stage II to this latest version ?
Is it only a software update that should be taken or should I replace and IC in it ?
And does it cost anything ?
Thanks !
The Stage 2 does not need a hardware upgrade, nor does it cost anything to update the software. 
Download the binaries and have a careful read of the files within. If you have any questions or worries, post here before doing it.
Software is updated over a Midi (DIN or USB) connection. As long as you a desktop or laptop with Midi capability or a simple USB cable, you will be all set. Depending on the version (I’ve lost track), you may need a SysEx downloader (free piece of software); but then again, the latest version might have it’s own installation app.
Note, you must update the boot loader, before updating the main OS from 2.x to 3.x. Be aware that you can not roll back without a usb debugger and opening up the LP. All the detail are in the zip file with the OS3 update on the phatty web page, along with an addendum for the manual.
There is also an update available for the “usb processor” which handles both usb and din midi for the stage 2 and slim. Contact moog support to get the file. That update was not officially released, because there was not a working mac version, windows only. It clears up a dropped/stuck note issue that existed from certain controllers/sequencers.