What is the transition like for a former AD officer trying to get back into the Guard while in grad school?

Guard Jobs and Slots are driven by what is available in the State. You will normally keep your rank going to the Guard even if it involves a branch transfer. I would look at the states surround the area of the Grad School you are attending and see if any have an Ordinance Unit or an SFAS. Then talk to Officer Accessions at that states headquarters. If not, looking long term, I would look at branch transferring to the Branch with the largest presence in your area as a career move.
I branch transferred when I joined the Kentucky National Guard. They had one Battalion of Infantry, but almost of a Brigade of Engineers and they were short Engineer Officers. I had to attend what was then Engineer Officer Advance Course to complete Branch Qualifications, but there is a Reserve version that allows you to break that up into two shorter Active phases with a distance learning segment in between.

I'm unfamiliar with officers, I know that enlisted keeping their rank depends on slots available.

You will keep your rank. Branch transfers are easy for junior grade officers. SFAS is definitely possible if your State has an SF unit. EOD is Harder to come by due to limited slots, but you can check with your respective State’s Officer accession folks. The key is to keep an open mind about branching. The Guard has great educational benefits.