Consider a scenario where you have an account with groups prior to account mapping. None of these groups are mapped to an environment group or stage. Let us take the below journey of migrating these groups to an environment groups and stages mapped account.

Let us start with creating two groups, Old_Success and Old_fail with the below selected criteria:

You will notice at this stage that the masthead is empty and does not provide any options related to environment groups or stages:

Now let us create environment group mapping with the below criteria:

Let us now select edit button against Old_Success and Old_fail groups and select Development stages for both groups:

The masthead filters are now successfully able to filter the Old_Success and Old_fail groups. Hence, these groups have been successfully migrated to the new mapping.