If you had the pre-Classic version (known as Opera Mobile) then Opera would install over it and put all you old bookmarks in a Legacy Bookmarks folder in speed dial, but they changed where "Classic" installs so that it and Opera for Android could co-exist.
The only way around that is to remove Opera for Android and install an old version (available from Opera's FTP server, ftp://ftp.opera.com/pub/opera/android ), if you have bookmarks in Classic then use Link to get them into the older version, and after that install a current version. You'd have to temporarily allow non-Market installs to be able to install the older version, but otherwise no big deal.