i thought about trying this....i have a 400W MH and a 1000W HPS....

but the connection to the refector from the 400 ballast is different from the connecter from the 1000 ballast.

so i dont know if this will vary in your case...or from 250 to 400....but it did for me from 400 to 1000...

...ill hopefully take some pics tonight and show you tomorrow.