Google-workspace – upgrade just one account to Google Workspace for Business

It is possible to change the way you validate a domain name. Proceed to Webmasters Central and there will be links to the verification details of every domain attached to your account (provided that it is the same account that added the domains to the system). Clicking on it will bring you to a page with more details on the domain. There is an option "Verify using a different method.", using it will guide you through verifying your domain using an alternative method.

No. There's no longer a free edition of Google Apps. If you had signed up for a trial full account before December 6 2012, you could have cancelled the trial and gone back to the free edition. Since Google no longer offers a free edition, and December 6th is more than 30 days ago, you cannot revert to the free edition.

From the Google Support article:

After the last day of your trial period, you can no longer downgrade to the Google Apps free edition.

You can cancel your account or delete your domain.

If you have not entered your billing information and you do not explicitly cancel, your account will expire. However, if you've already entered your billing information, you should cancel to ensure that you're not charged at the end of your trial.