unanswered Pokémon Black & White - NPC Overworld Sprite Changes cause Error

Hello all,

After replacing several NPC's overworld sprites on Route 5 in Pokémon White (by this I do NOT mean completely replacing sprites using the sprite editor, but simply editing a Baker into a Roughneck overworld using hex editing, for example), the game started to mess up in the following ways:

• the game freezes when trying to save on Route 5;
  
• the player's overworld sprite disappears when using the Bicycle on Route 5;
  
• activating the Cheren encounter script after battling Elesa doesn't show Cheren's overworld sprite (only its shadow);
  
• the same goes for Elesa when she appears to open Driftveil Drawbridge: only the overworld's shadow appears.
  

I found out that everything works fine when I replace less overworld sprites. Could it have something to do with the header assigning a certain amount of overworld sprites, or does it have something to do with the scripts?

It would be wonderful if somebody knows what's causing these problems.