You must deactivate the license on the dev site before you can activate it on the new site.
To do this you have 2 options.
1) Go to http://the-dev-site-domain/ and log in and dectivate that ACF license on the site. You must be logged into the admin of the domain where the license was activated.
2) Log into your account on ACF and deactivate the site license there.
