Site Plan Yearly Billing Upgrade

Hello, I’m new to Webflow and working on my first site. My client thinks he may need CMS but not sure how soon. If I start his site on the Basic Plan Yearly Subscription, and maybe three months into the year there is a need to upgrade to the CMS Yearly Plan, will I need to buy a full year on the CMS Plan or just an upgrade on the nine months left of the running yearly plan I already have?

I will appreciate your help with this question. Thank you!

Here is my site Read-Only: LINK
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