The feature where you can open the Levels window, select an actor from one level and move it to another worked for some actors, but not all for me. But I figured out a workaround. Open any level sequence, reference all the actors in the sub-level and convert them to spawnable; which empties out the sub-level. Disconnect/delete the sub-level in the Levels (I went further and deleted the umap, as well). Go back to the level sequence and convert all the actors back to possessable. It puts them back in the primary level.