I once had a class M in California but didn't renew it because I thought I would never get another bike. Can they look it up and maybe just take a test, or do I have to start all over again?

Your M endorsement has expired. You will need to pass the written and riding evaluations. You may also take a one-day advanced rider course at a local provider. You will use your own motorcycle for this training. Insurance is required. Proof of registration of the motorcycle is required. How you get the motorcycle to the training site is up to you. You can also take the 2-and-a-half day training that covers more of the basics. A motorcycle will be provided by the course. But, this course will cover many important things about driver/motorcycle awareness, which could be valuable in saving your life.