Entering the s/n into the older version does not work. I was told by my reseller that it is possible to downgrade. If pasted in a response from my reseller below. When I contact Adobe support, they point me back to my reseller. This is very frustrating. There should be a portal where customers can perform the downgrade themselves.
"Adobe allows CLP and TLP program members to order a current-version license but use a prior version as long as the customer already owns a license of the prior version. Qualifying members can contact Adobe Customer Service to request access to a previous version of perpetual licenses via ESD (if available) up to one version back. The program member must follow all guidelines of the current-version EULA."
We here on the forums are (for the most part) fellow-users, not Adobe Employees (unless clearly marked as such), so we certainly can't help you with it. I think that if the reseller promised you it should be possible then they need to help you achieve it, or refund your purchase.