Y instant to PN bucket recently
Yes, I have seen more Y instant upgrades to PN bucket recently, instead of JN bucket. When I try to get the upgrade through web, I can sometimes get one or two segments cleared. If there are FC seats available, I can get cleared if I call to get the upgrades for the remaining segment(s). My guess that that PN bucket is capability-controlled by SHARES. But CS agent has no trouble to upgrade Y tickets if there are F seats for sale. Sometimes, he/she needs to get to supervisor (more waiting time).
halls120 and QBK gave the correct observations. But YCA, VCA and DG fares are dependent upon city pairs. It has nothing to do with different government agencies. If a city pair YCA fare (IAD-SFO) is offered by a competitor, UA can offer DG (government discount) fare for this city pair. This is why the DG fare is identical to (or lower than) YCA fare. One cannot get Y instant upgrade with DG fare at UA. Hope that my explanation helps. Good travels.