Believe that it was; I seem to remember pre-merger CO Y/B (previously Y/H) instant upgrades and all other elite upgrades -- at least when processed automatically -- posted as "R" which made figuring out the original fare class sometimes challenging when it mattered (agent-processed upgrades were about equally likely to wind up in R as they were F IME).

Unfortunately at the moment I can only find a printout of my Northwest Account summary from 2006 (from when CO was in Skyteam and I was crediting all of my CO flying over there because the benefits were equivalent and I started as an NW flyer) and from late integration in 2012 (the tail end of account posting being so chaotic and unreliable that I'd print a copy almost every time it was actually correct -- I think this was the last one before I started mostly trusting the system again).

The NW shows my CO flights as "Class Flown/Paid" with "F/U", "F/K", "R/L", "R/O", "R/Y", etc. for the upgraded flights vs. "Y/Y", "O/O", etc. for non-upgraded flights and that matches my memories of the way CO displayed it after I swung over to OnePass (when DL/NW merged and CO would finally match my NW status)

The UA 2012 capture on the other hand shows upgraded flights as ZB, ZY, etc: