You need to unlock the phones if they are locked to AT&T. See-->http://www.att.com/esupport/article.jsp?sid=KB414532
You then need to contact Verizon with the IMEI to see if they will accept the phones for their network. Whether they can depends on the model number of the phone; some phones are not compatible with Verizon's network. One way to check is to go to Settings/General/About. If there is an MEID listed the phone is compatible with Verizon's network. But even then if the IMEI is not in their database the first level support will say you can't use the phone on their network. You need to be persistent and ask for your question to be escalated to Tier 2 support. These folks can add your IMEI to the database.
As the phones were financed through Apple and not AT&T that should not be an impediment.