Is it possible to archive (save) projects and downgrade the plan?

You cannot archive projects and downgrade the plan on Tilda, unfortunately. If you do not need some projects, you can only delete them from the account. To delete a website from the account, go to the Site Settings → Actions → Delete website. Then, to switch from the Business plan to the Personal plan, you need to wait until your paid subscription period expires and then buy a lower plan.

If you do not want to downgrade the plan, but would like to save the projects you do not need instead of deleting them, you can unpublish them. Such projects will be saved in the account but will not be available in the Internet. To do this, click on the Site Settings → More→ Unpublish.

Important: If you buy the lower plan before the subscription expires, the paid period of the current subscription is not recalculated. So you need to wait until your current subscription expires.