Can I switch to a new pricing tier?

Between the yearly plans. Of course. Contact support. Every plan is the same GuideGuide, so the only thing that changes is what you pay.

From a yearly plan to The Lifer. Switching isn’t an option. You can cancel your subscription yourself from the FastSpring account site and then make the one-time purchase once the paid period expires. Or buy it now and existentially wrestle with whether you’ve wasted part of a year or not. The end result is the same either way. There is no prorated path though.

From The Lifer to a yearly plan. You’ve embraced the existential path. You already unlocked infinity and yet still you wish to earn it. Might I suggest you consider The Benefactor plan.