The short answer
Yes. You can upgrade, downgrade or cancel at any time from Settings, then Billing, and there is no long-term contract. Upgrades are charged right away, with a prorated credit for unused time on the old plan. Downgrades are also self-serve, but HighLevel warns that they do not cancel add-ons, marketplace apps or phone numbers, and the lower plan's sub-account limit applies.
How to upgrade
Go to Agency Settings, open the Billing tab, click "Want to Modify/Cancel Your Subscription?" and then Upgrade. Toggle between monthly and yearly billing, pick the plan, click Upgrade Now and confirm the payment. The pricing page says that if you upgrade mid-month you are charged only the prorated difference. The billing FAQ describes the mechanics: an immediate charge for the new plan with a credit for unused time on the old one, which is why an upgrade can look like a second charge in the same month.
How to downgrade
Use the same billing screen, choose the lower plan, check the new price, the effective date and any prorated adjustment, then confirm. If the option is missing, contact support through the in-app help icon. Before moving from Unlimited or Agency Pro to Starter, count your sub-accounts, since Starter includes 3. HighLevel's billing article tells you to review the lower plan's limits first but does not say what happens to sub-accounts above the new limit, so ask support before you confirm the change.
Switching between monthly and annual
Moving to yearly billing is offered as part of an upgrade, where the billing screen shows a Monthly and Yearly toggle. Going the other way is harder: the billing FAQ says changing an existing yearly plan back to monthly is not available in the platform, so you need to contact support. Yearly pricing is ten times the monthly rate, which comes to $970 for Starter, $2,970 for Unlimited and $4,970 for Agency Pro.
What moving to Agency Pro changes
The step from Unlimited to Agency Pro brings the biggest feature change. At $497 a month, Agency Pro adds SaaS Mode, automated sub-account creation, rebilling of phone and email usage with a markup, user and agent reporting, and advanced API access. SaaS Mode is only available on Agency Pro. If you later drop back to Unlimited, those Agency Pro features go too, including any markup you were earning on client usage, so plan client pricing before you downgrade.
More answers in HighLevel questions. For costs, see GoHighLevel pricing or run the cost calculator.