sure, the question is at what price. if it's within the 30 days you may be able to return your existing phones and pay the discount for the upgrade to the subsidized price. you may not be able to all the same
we've seen stories of people who have to wait 2 years for the subsidized price again, others were 12, 13, 15, 18 months and anything in between. for me it was 11 months inbetween to be upgrade eligible
basically, call ATT and ask. they'll know about your specific account