I don't think that you can rename or change an existing Custom Step type without affecting the existing steps that you have created. Basically, as I understand it, you've removed the step type and the existing sequences need that support for the custom step types.
As for making changes to your existing Custom Steps, you can make changes to custom steps and have ALL OTHER Custom Steps UPDATE to match the changes by:
1. Open ALL files that contain the Custom Step that you want to modify.
2. Open the "Mytypes.ini" (or where ever the custom steps are located)
3. Open the custom step properties that you want to change.
4. In that custom step, put a check in the "Apply Change in this Dialog to all Loaded Steps of this Type".
Now, any changes that you make to the Custom Step will be propogated to ALL Loaded (Open) steps of that type of custom step type that is currently loaded (open).
This works great if you need to add another parameter, changes to the Pre/Post/Status expressions, or any other setting in the custom step.
Mike.