I believe Adobe offers free installation support. Give them a call. (We're just users here in the forums.)
Premiere never "looks" for an older version on the system when installing an upgrade, and in fact, the best thing to do is completely uninstall any old version before upgrading. Also, you should run the "Adobe Cleaner" utility to remove any remnants of old software before new install. Only an older serial is needed for an upgrade, but of course must meet version requirements.
If you cannot get Premiere to install, please contact Adobe support as recommended
Safe Harbor Computers
In the US - Adobe General support 800-833-6687 M-F 5am-7pm Pacific
In the US - Adobe Install Problems 800-642-3623
In the US - Adobe Activation 866-772-3623 Open 24/7