There are only two ways to make the change, either of which can be done online or by phone:
If you have enough points to make the AKV reservation first, then you can do that and then modify the THV reservation by dropping days. That is the safest way to do it.
If you need points in the THV reservation to make the change, then it is a two step process: (a) you have to first modify the existing reservation by dropping the days applicable too AKV; (b) you then make a new and separate reservation for AKV. The problem with that process is that there is some risk you could lose your reservation. You might see that AKV is open right at 7 months out, then drop days at SSR, and in between doing that drop and going back to do the AKV reservation some of the days you want become filled by others doing a reservation. Moreover, you may not then be able to top back and get the THV for those dropped days because those get taken by someone else during the time you are attempting to do the change.
Likely, because THV and Kidani 2BRs are usually open at 7 months out that time of year, you will not have a problem. However, particularly for the future, what you should do when you are considering a split trip is to initially make two separate reservations at 11 months out for your home resort, one for the days you intend to keep at your home resort and one for the days you intend to switch. Then, at 7 months out you only need to modify the second one to make the change and can do it online or via phone with no risk of losing your reservation. If what you want is not available at 7 months out, you can then have MS merge the original two reservations into one to avoid having to change rooms while at WDW.