I also am not sure why some horses will colic on coastal bermuda and others will not.
When I moved my three from the OH/PA border to Southern California, I only had room for a couple bales of their Pennsylvania timothy hay.
Being in SoCal, they lived in a sand area with absolutely no grass.
I took less than two weeks to switch them over to fluffly bermuda hay that was imported down from the Imperial Valley.
As far as eating forage goes, they ate nothing but that bermuda hay for the five years we lived there and nobody ever got impacted, nobody got sick nor colicked.
The only horses I have personal experience with that colicked on bermuda, were show horses that were stalled all the time. Their owners ran out of the really crappy dollar-a-bale hay they fed and bought a bale of bermuda to get them thru. With these horses, I THINK they ate too fast because the hay was so much tastier, plus it is not stemmy, and they couldn't digest it for gulping it down - at least that is my un-educated theory.
I still feed coastal bermuda to my 25 year old Arab because he has four molars missing and refuses to chew anything stemmy.
Once you start making the transition, watch their manure for any changes in moisture content and the amounts each horse presents you, if they spend anytime in the stall.
That is one of the prime reasons I bring my horses in at night. It's a pain cleaning stalls every day but I can look at their manure and pretty much see what's going on with their digestive system.
Hope this helps