There is some information in this thread.
The thread Nick referred to identified MindManager 14 as the last version that could read .mmp files and convert them to .mmap format. At that time MM14 could be accessed via the downloads library at the MindManager support website.
This access was subsequently removed and the downloads library now supports versions only as far back as MM2019. However, the MM14 download can still be accessed via the Internet Archive (Wayback Machine) using this link. Please note that as indicated in the original thread, version MM14 is unsupported and you would have to complete all the file conversions during the 30-day trial period.
Thanks Alex for clarifying :) I still have a running version of MM14 and could offer a bulk conversion service.