User Defaults - User Group Overwriting Existing Assignment

We are looking to use the "User Defaults" capability of GRC. Mainly to auto-assign parameters / user groups to end users as they submit requests. Currently in Production, we have user control coordinators who submit the GRC requests for end-users directly. These UCCs have knowledge of our user-groups and can assign them manually per the user they are creating. But going forward, we want the user-group to auto-assign based on the template selected. This way, the end users can submit the GRC tickets on their own and do not have to decide on a user group.

I have maintained the user defaults in SPRO and setup the proper BRF+ rule. User group and parameters are getting assigned appropriately in our test environment. However, I am noticing for existing users, their previous user group is getting overwritten by the new user default user group. This will not work for our needs as the existing user group should remain and not be affected by the defaults. Ideally, only new users would be given the user default user group and all existing users would retain their existing user group. Is there a spro setting or parameter I can set to accomplish this?