Opera 12 does have new internal mail format which is not compatible
with earlier versions. Fortunately, this new "format" seems only
apply to mail indexing and some mail INI settings. The mail message
storage, which are MBS files are not affected. So you can simply
move the mail store directory to a safe place, then import all the
MBS files into your email account (only do this when Opera is not
running). Of course, email settings may need to be reconfigured all
over again, especially for filters.