Gotcha.
I'd follow the directions in the second part of the error you received, the
"Note: You will need your Customer Number and either a Service Contract Number (SCN), Sales Order Number (SON) or Site Number in order to upgrade your account. This information is available in the Software Order Fulfillment email you received when the software was purchased. If you cannot locate this information, or would like more information around our Support Packages, please contact your company Administrator or a PTC Global Support Representative."
part. I assume you have those numbers mentioned readily available, but if you contact PTC Support that way they should help you.
(And maybe you can ask them about the licence management for your original problem, the Mathcad 15 install media!)
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