Change the capital

[Lord Finnish]

Belgrade is way too close to the frontier (which is one of the reasons Rome was abandoned in favor of Constantinople) and C-Pole is in a very good spot, is in the middle of the Empire (thus minimizing distance penalties) an is much more developed.

So besides nationalism, there is virtually no reason for you to keep your capital in Belgrade. But obviously it is you who decide your capital, not me.

 

[Heartsbane]

The others have made a good case for why Constantinople is the better choice but if you are already a strong state (and if you conquered Hungary and France that certainly seems to be the case) then why not keep it for sentimental reasons. You could always just make Constantinople your capital and wait for the culture to shift to Serbian though.