If you are not too happy with the food in the first place, I would suggest you change feeds rather than going off and then back on--you would have to slowly decrease their food, then slowly increase it back up when you get some more, and this really isn't the best option because, like you said, colic, but more gravely, founder.
So when considering new feed, make sure you still have some of the old grain left so you can slowly change it over. For example, if you are giving, say, one pound Horseman's Edge per meal (a total random number), and you are going to change to a grain called "X" (let's say)--start by only giving, oh, around 1/8th of a pound of "X" and 7/8ths of Horseman's Edge one meal, and still a full pound of Horseman's Edge in the other meal. Am I making sense?
Then start switching away from the other time of feed, as well, and go back and forth between which meal you are giving more "X" and less Horseman's Edge.
How long you wait between each switch depends on your horse's weight, breed, etc. If you have ponies/drafts I would definitely air on the side of caution by waiting more days vs less. It'll average out somwhere between 6 and 10 days for each 1/8th added to one meal.
If you're not going to have enough feed to last you over, then you'll have to go with the option of slowly dwindling away their grain. Just one thing I want to point out--it has very little to do with how MUCH hay they will be getting, but the QUALITY of the hay. If it doesn't have the right nutrients, adding more won't get you anywhere.
So I would definitely check that out.
Overall, medling with feed is an art AND a science. Figuring out which grain you're going to switch to is also a whole other matter. I would definitely talk to your vet on that one. Foundering is the biggest concern you should keep in mind while doing this. Again, always air on the side of caution.
Good luck to you, and feel free to ask any questions if I didn't make sense or left something out, ha ha!