They have until May 1 to select a new plan (of which there are several tiers), or Google will do it for them “based on what [they] currently use with [their] G Suite legacy free edition.” However, billing won’t start for at least two months if you were automatically upgraded.
Upgrading from your G Suite legacy free edition to Google Workspace will only take you a few short steps and is not disruptive to your end-users. To support you in this transition, you will have discount options for 12 months after July 1, 2022.
Plans start at $6 /user/month with Business Starter and go up to $18 /user/month. (Very small businesses with just one existing Gmail can also upgrade to Workspace Individual for $9.99/month, but will not get a custom email address.) Google will suspend your automatic Workspace subscription/accounts if you do not enter billing details before July 1, 2022."