It's not legal to suddenly change price if you had an agreement. That said, the agreed upon purchase would have to be legitimate.
Doesn't matter what a chat rep said if the promotion wasn't valid. For BIC promos, you have to meet 3 requirements. Purchase must be made during the promotional period, device had to have been part of the promotion and you'd have to meet any prerequisite such as a new line or trade-in (sometimes this isn't required - it all depends on the specific promo)
If you don't meet those, the price wasn't change, you simply didn't qualify. Now if you did meet requirements, it doesn't matter if the promo ended so long as the purchase date fell within that time.