Wait... two things intertwined in there.

Can I use the same ballast reflector (the 400W) for a smaller bulb?
You can use the same reflector.
You can't use the same ballast.
'ballast reflector' is confusing...

So, you can't plug a 250 watt bulb into a 400 watt ballast.

Hope that helps.

Another option is to buy a full ballast+bulb set of lower watts.
??