Do I need to go back to AIT to switch back to my secondary MOS?

This should not be necessary... Should being the key word.

However, I can think of two scenarios where sending you back to AIT would be the prudent path.
1) MOS re-alignment meaning a current 25B is not the same job as what used to be a 25B. I highly doubt this, and even if it had, I am pretty sure you would have already figured this out.
2) substantial advancements in the field meaning that in the 8 years since you last served in the MOS, the technical aspects of the MOS have changed so significantly that your previous training is no longer valid. In the IT world, 8 years is a lifetime.

MSG (Join to see) hits the nail on the head with producing paperwork to show you are already MOS-Q. But I would also ask current 25Bs about your knowledge base and if it is up to speed. Maybe going to AIT to "catch up" is what is best for you, your future Soldiers, and the Army.

You should not have to re-enter AIT for an MOS you already possess. What you need to do is find all your graduation certificates, 1059, and what ever else they gave you when you first completed 25B school. Once you have those, present them to your Training NCO....and perhaps Operations NCO.

I would go anyways. Possibly more up to date training. Take it as a refresher.