Try it. I am in zone 6 near Cape Cod and I am still harvesting sweet lettucce by cutting back old plants of black seeded simpson and Paris Island Cos. I must admit that my table is supplemented by area farm stand lettuce as well. In your area, I would suggest planting some new lettuce in a cold frame for fall/winter harvest. Sometimes Jack Frost beats you in the game but it is a lot of fun playing.
Good Luck

Yes, I definitely have cut bitter lettuce to the roots (just cut off, no need to dig up and replant) and gotten sweet lettuce growing from the old plant. Especially now that the weather is cooling down, this could work very nicely. Watering well reduces the bitter factor, too.