If a directory-service aware domain client changes their password, the change is immediately replicated to the PDC FSMO role holder by the validating domain controller, even across a WAN.
If you prefer to delay this WAN replication to the normal interval, use Regedt32, on the validating domain controller, to navigate to:
On the Edit menu, Add Value name AvoidPdcOnWan, as a REG_DWORD data type. Set the data value to 1.
The password change will be replicated to the other domain controllers using the normal replication interval.