OP actually said because the meeting will go long, he will not make his flight. So his best option that's not violating the CoC is to do the ticket change with the TA/airline and suck up the fees/fare difference.
You forgot to read/quote the rest of OP's post. which explains the purpose of the post here (bolding is mine):
Originally Posted by
digitaldilemma
I can currently purchase a one-way return the next morning that is one-third the cost of changing my R/T ticket. So this would be my back-up, insurance policy. There's always a chance that the meeting ends early, and I can still get out in time, but just in case, the one-way fare the next day makes sense.