I've just discovered that the Trade distance to foreign CoTs may actually change. I was playing as Navarre and trading through Novgorod, when suddenly Novgorod became too far. I should mention that:
1.) My Trade Range hasn't changed. I've reloaded a old saves and its still at 204, the same as after I lose access.
2.) Novgorod's CoT is still in Novgorod.
3.) I always lose access to the CoT on September 28th, 1410, even after reloading from the 24th of September. No popups.
4.) Trade distance to Novgorod is now 254. I don't know what it was before, and there seems no way of finding out, other than that it must have been below 204.
Anyone got an idea of how trade distance might change?