Not sure since inception - but there have been quite a few changes. Mainly documentented through the SB's. The key feature changes can be determined from the user
guide editions. And also the service
alerts.
However each installation has many different mods. I have a G reg C182 and have applied several SB's, some of which have been safety related and a few have been feature related. The features which are NOT ordinarily retrofittable are:-
1. WAAS (EGNOS) not needed for non-precision approaches. Have not had a satisfactory reply as to the possibility of an upgrade (it should be simple replacement of two units - but approvals seem to be the barrier.)
2. New Integrated auto-pilot - ideal for integrated VNAV but many prefer the KAP140 fitted to most installs. This would need to replace both panels and a hidden control unit with major wiring so too expensive to consider..
Other upgrades (with some cost)
1. Jeppesen MFD Flight Plates. (Currently IFR only)
2. TAWS-B for those likely to go CFIT flying. The basic terrain awareness is very useful for the average pilot (it is a license key the s/w is built in)
All the other fiddle and twiddles are in the manuals. One of the hidden twiddles was a change to the KAP140 to reduce "wander" in some installations.
Mine is having another s/w upgrade fitted as we speak which comes with yet another manual (arrived a few weeks ago). When I pick it up next week I will have a read and see what they have changed.
Was there anything specific you were after??