Looking at the ingredients listed for the supplement, it seems like a lot of money for essentially flax seeds, chia seeds, black sunflower seeds and some papaya extract: $82 for a month’s worth.
@Simkie is right: get your horses situated and their hay tested and then see what they need. But, you might consider weaning them off the supplement over the course of the first month just to ease the transition to a different feeding regimen.
Edited to add: if you look at the list of articles listed at the bottom of the supplement’s webpage, they seem to all refer to human studies about the importance of lutein and Zeaxanthin in the diet or one study references the usefulness of lecithin (also an ingredient) on the prevention of ulcers in horses taking bute. It’s a weird group of articles for a purported digestive supplement.
For example, here is one: Mozaffarieh, M., Sacu, S., & Wedrich, A. (2003). The role of the carotenoids, lutein and zeaxanthin, in protecting against age-related macular degeneration: a review based on controversial evidence. Nutrition Journal, 2 (1), 20