You can also, remove the inherit from a group. The policies that are in place will remain the same, as they had already been inherited from the parent group.
From here, what you can do, is next to the rules you want to assign/change, you can click on tasks and edit. A prompt will ask you, since the original was inherited, if you want to edit shared- meaning all policies, even though you are not sharing or edit non-shared. By editing non-shared, only the policy you are modifying in your group will be changed.
If you choose to keep the existing name of the policy that you are modifying, I beleive the "new policy" will be created (in the policies tab on the left) and will be given the name of the group it was created for- by default.
By going to policies tab on the left however, and making a copy of the existing policy and than giving it a name, you can choose to assign this policy to a group. Because of how SEP is designed, you cannot have 2 of the same Type of policy in a single group.
For example, if you have FW_Policy1 assigned to a parent and create FW_Policy2 and assign it to a child group; you would not be allowed inheritance of all the other policies to the child group. Thus inheritance would need to be turned off to assign the policy.
There is no advantage etiher way, as it will create a policy in the same location as your other, already existing policies.
The disadvantage, is the need to modify multiple policies in order to accomplish certain tasks or make "global changes" if you have many individual policies assigned to non-inheriting groups.
Making changes through the policy tab however, will modify all the groups assigned that policy even if "inheritance" is not set. Because the policy is assigned to multiple groups.
Sorry for repetitiveness and redundancy in here. Tried to read over, but definitely need of a coffee this morning.