According to a conversation I had with an engineer for CBS Radio in New York, they will not be upgrading any of their stations to -14 dBc anytime soon because it isn't a high priority for the management. In fact, they did receive a new transmitter for one CBS radio station in NYC that could do -14 dBc but that transmitter had to be sent to another station to replace a failing transmitter, so CBS New York has no transmitters capable of outputting -14 dBc HD. I suppose the same will be true for Philadelphia since all the CBS stations adopted HD early. The management is actually smart enough to realize that there is 0 return on investment for the -20 dBc HD transmitter, there is no point replacing a perfectly good transmitter just to blast 6 dB more IBUZ with no return on that investment for at least several years, if at all. Unless you just bought a new transmitter, increasing HD power isn't as easy as changing the output power on the transmitter.
The best answer to your question is "never".