It will be fine in the sense that you will be able to use it. But remember you'll lose a lot of reach; 70-200mm on a 6D has the field of view of 44-125mm on your 40D. You might want to think about whether that's enough for equestrian photography.Well i've just purchased a Canon 70-200 F4 L for Equestrian/Sports photography which should be fine.
The good thing is that the 6D being Full Frame and twice the pixels on the sensor as the 40D has enough latitude for cropping if the 200mm isnt quite enough reach. The 320mm equivalent at the 200mm end on the 40D is basically a cropped image anyway. Its still a 200mm focal length not an actual 320mm.
I went from a 40D to a 6d.
Great choice, and particularly at a higher ISO makes a world of difference.
I also have a 70-200, but have a 1.4 converter if I feel I need the additional reach (I don't do equestrian, so don't know if this is suitable).
The quality of the images is SO much better. I have kept the 40d as a backup, but the number of times it has been used is minimal.
Remember, you will have to get SD cards as well if you move (not that it will cause too much financial hardship).