Hyde here from Shopify. Great question!
If you opt for the annual plan but decide, for whatever reason, that it is not the correct plan for you, then you can downgrade to a monthly plan. If you do so within 30 days, a pro-rated amount will be credited to your account.
The option is not available within your admin, meaning that you will need to reach out to our support. If you go to this page and click Contact Support you will be prompted to either login or continue without a store. Once you have selected an option you will be asked to ask about a topic. Search for your query and relevant help docs will populate as well as the option to proceed to live chat or email, where you can request that your account be moved to a monthly plan.
I hope that helps!
All the best, Hyde.