All LAN PORTS TO WAN PORTS

Yes, but what such firewall could be used for?

A customer has six ISP links and wishes to migrate them to a new FortiGate firewall. I need to confirm the feasibility of this migration and determine which FortiGate model is suitable for the task

Hello Akshay289,

Yes, you can do that using the following article:

https://community.fortinet.com/t5/FortiGate/Technical-Tip-Configuring-LAN-port-as-WAN/ta-p/272378

Make sure you have at least one LAN port. Making every port on Fortigate as WAN doesn't seem to be a good idea.