I think you should be fine.  When you buy new timothy, it is likely from a different source than your previous purchase, so your buns have had different varieties before.  I know that certain veggies can upset their system if they are not used to it, but I have never read that about hay.

Hay is usually just fine, especially when it is the same type, i.e. timothy. If you think about how crops are grown, they get different versions of hay from the same field all year long, depending on the cutting. Some is really sweet, others aren’t. The thing to worry about is dust, weeds, seeds and mold. But if you buy it by the bag in a pet store, it is usually in pretty good shape as opposed to a bale that you really need to inspect closely. Do you have any feed stores near you or have a friend with a horse that would sell you a part of a bale? You can save a lot of money that way. Wait, come to think of it, didn’t you say that you have a horse? The same hay that you feed them is just fine for a bunny too.  You definately don’t want the stuff that cows are given, it is a lower grade and can contain mold. Some places will say there is no difference and they sell just one kind but there are places that do so be careful!

Oh dear, well it is better to be safe than sorry. Some hays do have a lot of that gook in them, I’m fortunate to where I have a feed store close by that sells very clean hay as far as weeds and other non-hay plants go. So sad to hear about the two bunnies you have had that passed away from GI stasis, I would be very careful myself too. If I might ask, what kind of diet are you feeding your bun now?