While waiting for this idea to be addressed, I use this GroupEditor class to manage the ungroup - change - regroup process.

The GroupEditor allows to ungroup one instance of one GroupType, so the user or the APIs can modify the group member entities, delete or add some. When the changes are finished it is possible to regroup all the entities into one new group and assign the new GroupType to all the other instances of the old GroupType.

Use GroupEditor.StartEditing() to ungroup the group and create a DataStorage containing the list of group members and other group properties.

Use GroupEditor.AddElements() and GroupEditor.RemoveElements() to add and remove some elements from the list in the DataStorage object.

Use GroupEditor.FinishEditing() to regroup all the elements. If more instances exist for the same GroupType, they all are updated to match the newly generated group.

After using GroupEditor.StartEditing(), it is possible to give the control back to the user interface and call GroupEditor.FinishEditing() later, even after saving, closing and reopening the project.

Unfortunately tons of the group properties are lost, like reference level, excluded entities, some constraints (not all), etc. Some of the properties are lost because Revit APIs do not manage them, some are lost because I didn't need to manage them yet.