Okay, so I'm switching Reesie over to Orijen and I started him with 25% of the Orijen and 75% of his old food (Wellness, which he is bored with and does not like to eat).  He loved the Orijen and was very excited about eating it.  The only problem is, he picked through all his food and 
ONLY ate the Orijen and left the rest of his old food. (Not that it's old food, but it's what he has been eating, you know what I mean. LOL!

)  Anyways, I've always heard it is bad for their bellies when you switch their food instantly over to the new stuff.  What do you think I should do?