Assuming you're talking about the firewall policies/rules.
If the change you want to make is something like adding a tos value or add logging then is possible. In the CLI when you're in the Policy, just rewrite the rule adding the new parameter and be sure to specify the position. The should rewrite the rule.
If you want to change 'source dest svc action' then this may write an additional rule, though I haven't tested that.
:smileyhappy: