Welcome to the Community - There are two components that need to be licensed - the ESXi host and the vCenter server for you to have HA and vMotion - the vCenter Foundation license you bought is only for the vCenter Server and the Free License can not be managed by vCenetr which is required for HA and vMotion - so your options are to but the approriate licenses for your ESXi hosts or as Andre suggested return the vCenter Foundation for the Essentials Plus kit which is vCenter and 3 ESXi license bundled together -
Also fo not worry you will not have to reinstall the tro ESXi host it is just applying the new license -