You can convert a classic Firewall policy to advanced. However, you cannot convert an advanced to classic.
Steps:
Select Intrusion Prevention → Policy Types → Firewall.
Double-click the required classic Firewall policy.
Make sure the Editable here column displayed as Yes for the policy. If not, go to the domain which owns the policy to convert it.
On the Properties tab, select Advanced from the Type drop-down.
Click the Access Rules tab to view the corresponding rules.
The additional parameters applicable only to advanced Firewall policies are now available for these rules.
For each access rule, review and specify the required criteria.
Do a configuration update to the corresponding Sensors for the changes to take effect.