After a couple years in a toxic boarding environment, I'm finally moving my horse to hopefully a better environment. Pros and cons to every place. But he will be going from out on pasture 24/7 to a dry lot with free choice hay. He will have access to a stall but can go out anytime he wants. 
Will I need to supplement with a certain supplement because he won't be getting as much green grass? I'll still graze him but I know it's not the same. 

Also wanted to get an opinion on if I should change his feed or not since I will actually have a say in what I feed my horse now. He is HYPP N/H and knock on wood he hasn't had a flare up since I've owned him in 4 years. He's currently on Safechoice Original grain. 
His supplements are currently: topline balancer, purina outlast, an oil and succeed. 

Feed wise I would keep him on what has been working. The new barn owner should be willing to work with you since your horse has special dietary needs. 

Going from pasture to dry lot could be a hard transition, however, if your horse will truly have access to hay 24/7 it hopefully wont take your horse too long to adjust.

I take my gelding to the new barn this afternoon! I'm nervous but I'm trying to make this as smooth as I can for him. I do feel horrible he won't be getting much grass, but I'm doing my best. The new barn owner seems like she will be accommodating!

Should be ok with going from grass to free choice hay, I wouldn't change your feed unless there's a weight change. And always check for ulcers first with the stress of moving. I would just be mindful of the type of hay, hopefully its grass since they shouldn't have alfalfa. I would say thats your biggest concern really 

Yes she feeds grass hay. He was a bit nervous moving yesterday but settled in nicely. He's enjoying having shelter out of the baking sun and deer flies! Where he was before, they had no shelter really. He can go in and out at he pleases. He can touch noses with a few other horses if he wants. He's by himself in his dry lot which is attached to his stall, but he has other horses right next to him.