You can upgrade immediately because we'll pro-rate your new plan with the credit you have left on your existing plan.
If you're in the yearly plan for Premium Automation, you'll still be prorated according to how many days you have left on your current yearly plan. We'll credit this towards your Ultimate plan. If you have a lot of days left, it's possible you don't have to pay for a few months.
You can adjust your plan by going to "Setting > Account Setting"