SNAT of source IP based on destination port

If the traffic for which you want to change the Source IP is handled by an LTM Virtual Server, then one way to do this is to create and attach a snat pool to the virtual. You can also do this via irules (or possibly Local Traffic policies in the GUI), but you only need to do this if you need to apply the SNAT selectively based on some logic, because the virtual server will snat everything to the selected pool of addresses unless told otherwise by irules or LT policies).

If the traffic is not handled via a Virtual, but is rather device originated traffic (such as config-sync traffic) then you have to change the self-IP of the corresponding VLANs through which the traffic exits.