My horse currently is boarded at a barn where he is in a paddock with virtually zero grass (stubble so small I don't even count it), due to his inability to eat the coastal roundbales in every other pasture. Due to this absence of fresh forage, he receives currently: 
1 flake perennial peanut hay (the Southern version of alfalfa--nutritionally the same)
1/2 flake of orchard
1 scoop (about 4 lbs) senior feed 
TWICE PER DAY

I'm moving him to a place with HUGE, LUSH pastures. Therefore, his feed needs will be drastically reduced. In fact, new place doesn't really feed any hay at all until winter when the grass dies down. They also claim to feed very small amounts of concentrates due to the large volume of grass. 

How do I start him over there? He will be on the new grass 24/7. Do I give him just a flake of peanut to start? A reduced portion of the senior feed? I don't want to overdue it, and I don't want to starve him either. LOL

What would you do? 

Biggest thing to do is make sure he's not thrown out to the pasture 24/7 to start with. Thats more important than what you do with the current feed. He shouldn't be on it more than 20-30 minutes to start, so you will have to maintain his current feed. As he gets more and more pasture you can reduce the current feed. I would pull grain away first, then forage(hay). You already feed a super small amount of hay anyway, it won't make that much difference. Unless you're talking the giant square bale flakes and not small standard square flakes. Once he's on full pasture 24/7 do bloodwork to make sure his nutritional needs are met. If he's a barrel horse, he will have different needs than a trail horse, pleasure horse, etc. If that need for extra nutrition arises, add in a multivitamin or ration balancer. It should take you 2-3 weeks to get him on pasture 24/7 at least 

Thanks. I have one of those grazing muzzles that has the 1" hole by the mouth, so I can put that on him to transition him onto the grass slowly. I'll do that. I'm thinking I'll also stop feeding the peanut hay for now, as that is so high calorie. I'll offer the orchard, but I'm not sure he will be interested with the option of grass. Tricky transition. Thanks for your insight.