Hi. There's no 'official' process - as far as Evernote are concerned Legacy was end-of-life years ago. AFAIK there is no physical way to keep the Legacy app functional after you lose access to the servers. You could try to stay logged in, but if your connection is interrupted for any reason you lose access to the app and effective access to your notes.
The EXB file format used to be viewable via a SQLite client so you might be able to rescue some information that way, but edits and updates will be difficult. You could export your notes to PDF or HTML files and try to construct a working reference database - but again all maintenance will be manual. The only two viable options seem to be:
Export notes and migrate to another provider or
Accept v10 with all its perceived faults and try to develop work-arounds.
At least you have the search / edit / add new notes options catered for, and have the time to develop work-arounds or find alternatives.
EDIT: (NB your existing database is still stored on he server and available via the web access)