Does anyone know if Adobe will allow me to have a Lightroom 5 serial number instead? The product has been paid for after all.
It may be possible, but you'll have to call Adobe:
Monday—Friday, 5am—7pm PT
Backward Licensing Policy
Adobe allows program members to order a current-version license but to use a prior version. These members can contact Adobe Customer Service to request a serial number for the earlier version if they do not already have one.