Without understanding why the vet wanted her off senior feed, it's really hard to make a recommendation. Truthfully, the first thought to come to mind is if she was holding her weight well on senior feed then that's what she needs to be on...especially if she is in her late teens or even older.
Sorry, but answers are dependant on the whole picture. Like when she was colicing, how much turnout time was she getting and how much hay (by weight)? Because the change in the incidence of colic may have more to do with the amount of forage she's getting than with the removal of senior feed. And how much water was she drinking before? Has there been a change in the way that water is offered/supplied? Was she in a sandy enclosure?
Licensed Veterinary Technician