There is no need to repot or transplant your non stop begonias. They are annuals, live just one year and do not mind being crowded into a container. The black spots on the foliage could be a leaf blight of fungus from too much cool, wet weather. Do not get the foliage wet when you water and try to move the plants out of the rain. If you want to save the tubers or roots to plant next spring wait until the top growth dies in November, dig the tubers from the soil and store in a brown paper bag in a cool dry spot. Cut off the top growth that clings to the tubers. You can try to replant the tubers in May but it takes awhile for them to resprout and bloom so buying a flowering plant from a nursery every spring is usually more practical. Your begonias love the shade. Pick off faded or dead flowers or any leaves showing black spots. Now enjoy these amazing plants all summer. Keep growing, Marianne Binetti