Pat,
I assume you will have to buy new ballasts (it may be time for a bulb change so that cost is negligable), will the $ saved in energy make up for the additional cash paid for the new ballasts?
If anything, I would suggest shortening your photo period. 400s run for 5 hours use the same energy as 250s run for 8 hours. There's a theory out there that corals have a finite amount of energy they can create during any given photo period and that this amount of energy is acheived in a shorter time frame than we leave the lights on (I believe you do this already with 2 4 hour time periods). You may find that running the 400s for 5 hours will be enough light/PAR output for the corals to thrive. Check out the TOTM, he rums his 400s for only 5 1/2 hours per day and leaves the VHO actinics on longer for viewing.
Just trying to offer a different view. HTH.