Implementing CustomField Validation

I’d like to add (?#Date)^\d{2}/\d{2}/\d{4}$ as a validation for a custom
field in one of our queues and am wondering what will happen for historical
tickets that don’t resolve to this string.

What will happen when a bulk update (say a priority change) is done on a
ticket with a value out of sync with this format?

I haven’t seen anything in the mail archives describing what to expect when
you add a validation string to a field that’s been in play for several years
worth of tickets so am hoping someone has an easy answer!

Thanks in advance!