Garmin just answered my question here... Thanks Guys!
We will once again provide free software upgrades for Garmin Mode S transponders (and the GTX 327) used in Experimental aircraft at Oshkosh.
Just bring your GTX 327, 23ES/33ES/330/330ES, 35R/45R/335/335R/345/345R transponder in to us at the Garmin tent at Oshkosh and we will install the latest software into your transponder while you wait. It only takes a few minutes to accomplish this update.
Here are the latest versions:
GTX 23ES/33ES/330/330ES: V8.04, Jan 2018
GTX 35R/45R/335/335R/345/345R: V2.12, June 2018
GTX 327 V2.10 software was released in Feb. 2014 and is the latest software for all GTX 327 units manufactured before July 2014.
GTX 327 V2.12 software was released in July 2014 to support newer hardware in the GTX 327 units manufactured after that date.
GTX 327 V2.13 software was released in Aug. 2017 to address an altitude reporting issue only when using a Gillham based altitude encoder with GTX 327 (011-00490-00) serial numbers 83750922 to 83752847 or GTX 327 (011-00490-10) serial numbers 84000356 to 84000357.
We are located across from the EAA Welcome Center and the Ford tent. If you look at the Airventure grounds map, this is grid K13. From homebuilt camping, just follow Knapp street South to Celebration Way (the main entry street) and turn left.
Sorry, but we are not equipped to perform sign-offs for certified aircraft, so please only bring us transponders that are installed in experimental aircraft.