“If you have a reputable car insurance provider, switching coverage from one car to another shouldn’t be an issue, even if you forgot. Just call your car insurance company, tell them what’s going on, and they’ll usually be able to make the change almost instantaneously.
In the off-chance that your auto insurance provider is hassling you about it, you might want to think about changing providers. You never let the insurance lapse, you were honest, and you didn’t miss a payment. If they’re telling you that the rate is going to go up a ton, try a different insurance company.