Modifying Mesh Sub-Objects

Mesh sub-object selections are preserved in the modifier stack. When a mesh sub-object level is enabled, the sub-object selection saved at that level is passed up the modifier stack. If you change the level of selection, the selection made at that level will be passed along instead.

If you want to make a new sub-object selection at any level without losing the old selection, you simply apply another selection modifier and make a new selection.