How can I go about swapping ECUs from a 05(mine) to a 04?
Are they interchangeable?
and Is there an easy way about doing this instead of switching out the cylinder and keys?

I suppose it would be easiest if I were able to retain my key and they keep theirs..

 

First of all, what's the benefit?

Second, it requires the immobilizer to be reprogrammed if you want to keep your keys/locks. Only the dealer can do that.

 

Aem ems

 

yes you should be fine