Probably the first commercially successful re-engining program was the Convair 580 (Allison 501 in place of R-2800s). 20 years later it was the DC-8-70 series (CFM56 vs JT3Ds)). In both cases over 100 ships were reworked.
One project TWA was considering in the mid-70s was to refit their CV880s, removing the straight-jet CJ805-3s with JT8D fans. Never happened, although some detailed engineering was underway.
LH at one time proposed reworking their 737-200 fleet a la 737-300 w/ CFM56s, but it was a bridge too far. Likewise Spantax wanted to "upgrade" their CV990's, replacing 4 CJ805-23 fans with 2 CF6's. (I wonder how long the MLG extensions would have to be?)
Similarly Delta and All Nippon threatened to refit their L-1011s with CF6-80A engines, and if they could have gotten TWA on board, it might have happened. DL and NH both had CF6-powered 767s coming on board 1982-83, so there was a commonality factor to consider.
