Conrail bought 145 new locomotives from GE in 1983-1985 (60 B36-7 in 1983, 50 C30-7Z and 10 C32-8 in 1984, and 25 C36-7 ("C37.5-7.5") in 1985). They used many of their of their U-series locomotives (other than U23B and U23C) as trade-ins. (At least the 1983 order was "worse" than 1-to-1: Conrail got, I think, either 3 new B36-7 for 5 trade-ins or 2 for 3.)
Very few U-boats from before the Penn Central merger would have survived this period; any that did would have gone as trade-ins for the next few Conrail GE orders.
There used to be a magazine called "Rails Northeast" (it had started as "Penn Central Railroader," and changed its name and broadened its coverage at the Conrail startup) which covered locomotive retirements in its news roundup. The U25B fleet had started to go a bit earlier (some perhaps in the very late 1970s). I remember noticing that ex-PRR U25B retirements vastly outnumbered ex-NYC ones in the first year or two of Conrail's U25B retirements. At the time I thought this might reflect PRR's lower maintenance standards, but it could also simply reflect the fact that many of PRR's U25B were a year or two older than NYC's.
I think I remember the C32-8 having been referred to as "C31-8" in early news stories about GE's new line: apparently the extra electrical equipment (notably two more motors) on the six-axle unit soaked up an extra ten horsepower in parasitic load: the B32-8 was announced as 3190hp and the C31-8 as 3180. Unless I am more senile than I realized.