How to prevent a old promoter region from attaching onto a plasmid instead of a new one during ligation?
Run it out on a gel after digestion  and extract the vector only. 
Treat with a phosphatase after digestion. This will prevent religation since you need at least one of your fragments to be phosphorylated for the ligation to work.
(A must regardless) sequence several colonies at the end. 


I recommend doing all three.

Edit: option 4: digest with a third enzyme that cuts in your old promoter but not the vector or the new promoter.