Thanks so much for all your suggestions Loose - I took them quite seriously and called my vet to talk about it. I told her I wanted to be safe because I'm afraid of them foundering on the new grass.
The vet put Tank on a 1-5 body condition score as a 4, she explains it as "fat covering ribs and pelvis requiring firm pressure to feel; e.g., an easy-keeping, mature horse on pasture with little or no work."
She put my pony as a 3 which is "ribs and pelvis covered with fat and rounded" and healthy.
I asked her if she thought the increased pasture would make it so that I should take them off grain, she said I could take them off corn if I cared too, since it's really just useless starch. But she said unless they gain any weight to keep them on the oats and alfalfa. She said my mare actually does have low muscle tone and I need to work on that with her, even if it's just in-hand working. But her low muscle tone makes her looks chunkier and fattier than she actually is. She said to keep the pony on grain as I plan on teaching him agility and doing packing with him, and he's a good weight so she doesn't want me to change it.
She said the crimped oats and alfalfa pellets are very good for balancing their calcium/phosphorus ratios and supplies them nutrients my pasture just doesn't offer. But she did say my mare could deal with going down on the quantity of grain I give her, so I'm going to put her on:
1cup crimped oats, 1 cup alfalfa pellets + brewer's yeast and MSM, twice a day
My pony she wants me to keep the same at:
1/4 cup crimped oats, 1/4 cup alfalfa + brewer's yeast and MSM twice a day.
Thank you very much for your insight :) I'll go with what my vet suggests but I'm glad I researched and feel confident their diet.