Okay, i understand the issue!
I think there was an WordPress version checking functionality, or there should be some old functions of WordPress.
May i know what is error message is showing when you try to update your WordPress?
Actually, I have to see the plugin code first, then i can assure you that whether i can do it or not!
But i am very much experienced in PHP-MySQL, WordPress Plugin development and Custom Applications development.
That's why i am really interested to solve the issue and provide you the best solutions.
Thanks, Waiting for your positive replay!