Moving up or down a tier takes a couple of clicks, and when it happens depends on which way you're going. Go to Settings → Plan & billing, find Change plan, pick Monthly or Yearly, then choose Switch on the plan you want and confirm. Max is monthly only, so it only shows a Switch under Monthly.
An upgrade takes effect straight away. The confirmation shows what you're charged today. You keep the usage you have left, and the extra monthly usage of the bigger plan is added for the rest of the cycle. From your next billing date you get the new plan's full monthly usage.
A downgrade waits for your next renewal. You're not charged today, and you keep your current plan, its monthly usage and everything else on it until the cycle ends. The smaller plan's monthly usage starts from that date. Until then the plan you're moving to shows the date it switches, with Undo next to it if you change your mind.
Usage packs don't change either way. They stay on your account and keep their expiry date.
Nothing is deleted when you move down, but a smaller plan can leave you over its limits. If it allows fewer Series than you have, the ones you've got stay and keep posting; you can't add another until you archive one, and archiving can't be undone. Dropping to Starter means no motion videos, only stills, and no High image quality on Lite, since those are on Creator and up.
Switch is off while a payment is unresolved, or while your plan is paused or set to pause. To compare tiers before you move, Plans and limits at a glance has them side by side, or email [email protected] and a human will read it.