Word is the MH is better for veg growth and is a whiter/bluer light whereas the HPS is redder and is better for flowering. I can't remember the science aspect of it, but I'm sure you've read that a million times yourself.
I have vegged with HPS and flowered with MH. Was there a huge difference either time? No. Did I even notice a difference at all? To be honest, no I didn't.
Used to have say 6-8 lights in a room. Each light had perhaps 15 plants under it. We had SO many rooms and so many lights that we didn't even pay attention to which lights went where. I mean we did, but we didn't stress over it.
This was in the day before digi ballasts so it wasn't so easy to lug around and change. Plus we both were at university full time and worked 30 hrs a week.
From what I remember there was no noticeable difference. I'm sure someone who knows more would dissagree, but I wouldn't worry.
Since you spent the $ you may as well use both bulbs so keep the MH going, but don't think you've made a terrible mistake. I'm sure many many ppl have a non digi ballast and just use HPS the whole time with great results.
For a first grow you're doing pretty damn good, don't stress.