So you're wondering about just changing bulbs? Can you run a sodium on an MH ballast? I know some ballast are like this but not all of them. Here's the answer to your question - don't use MH. You can grow just fine using only sodium and if you want to throw in some white light, then you use some flouro. It's a lot cheaper than MH and runs cooler and pulls much less power. Sodium is suppose to be cheaper than MH as well. Since it looks like you already have an MH ballast, just grow with that until finished. I don't think you'll see much difference. Just use 12/12 and should be fine. But in the future if you want sodium, don't even bother with MH anymore. That's my opinion. BTW, plants in flower continue to grow, so it's no surprise. Let them grow until the height becomes a problem, then trim.