Hi, Lisa!
To clarify, you do not need to change your Shopify plan in order to upgrade to Shopify POS Pro. The page linked in this post includes step-by-step instructions on how to manage your Shopify Point of Sale subscription. Shopify POS Lite is included in all plans. You can view the current pricing for Shopify POS Pro based on your plan here (click the + Full list of features button to expand the information): Shopify Pricing.