System and method embodiments are provided for optimizing offloading wireless services to a WLAN in 3GPP-RAT mobility. The embodiments enable changing network priority based on changing network conditions. In an embodiment, a method in a user equipment (UE) for offloading wireless services to a wireless local area network (WLAN) in a Third Generation Partnership Project (3GPP) inter-radio access technology (RAT) handover includes receiving an access network discovery and selection function (ANDSF) policy from an ANDSF server, wherein the ANDSF policy comprises policy rules including a validity condition, wherein the receiving is performed during a procedure in which the UE conducts an inter-RAT handoff from a first 3GPP RAT to a second 3GPP RAT; comparing evaluation condition information for the second 3GPP RAT against the validity condition; and offloading a first wireless service to the WLAN in accordance with the ANDSF policy when the validity condition is satisfied.