I've tried opening up a column in the schedule but it won't let me modify the cell (it is locked)
In the latest version of Synchro (18.104.22.168):
1. From the Fast Options toolbar, set the Task Selection mode (last drop-down on the right) to Task with Assignments. In this mode, when you select a Task, all the resources assigned to it are also selected simultaneously.
Or, if you want to edit the Appearance Profile for some but not all assigned Resources, leave this mode set to Task, and select the resources you want to edit the profile for.
2. In the Gantt window select all the tasks where you want to edit the Appearance Profile
3. In Task Properties>Resources, use the drop-down by Appearance Profile to change the value for selected resources assigned to selected tasks.
4. A dialog will appear reporting how many assignments will be changed. Press Yes
In older versions (in older versions there is no way to change the appearance profile for a subset of the assigned resources only)
1. In the Gantt window select all the tasks where you want to edit the Appearance Profile
2. In Task Properties>Resources, use the drop-down by Appearance Profile to change the value for all resources assigned to all selected tasks