I don´t see this as a problem at all.
To improve efficiency of their army and reinstating a historical place as their capital are in fact good reasons to change capital.
Besides you have a city that is in the center of their realm, close to most regions, something that can increase control over their regions.
If roleplayed correctly there is no problem at all on this. They have all the reasons to make Partora the new capital. It is strategy and history combined that are great motives for the change.
The problem is: if on the next war with their troops attacking north instead of south, and they change their capital AGAIN back to Perdan, then it will be an abuse of the mechanics.
