It's a toughie and I am currently struggling with it. If you operate your 707s on high efficiency with short TATs, it can be hard to change.
I think the best approach is to stretch TATs where possible and go to 75/76 on some routes, and A310 on others until the 787 comes along which is the real only alternative that I see.
The A321 has too shorter range, the 757, 767 take too long to turn around, and there is little else on the market that is the same in terms of range/ seats/ TAT.