Tetra-WebBBS Support Forum
No reason to loose any message from any prior version .... as long as the message datafiles still exist (sitting in /bbs<number> subfolders or the forum database).
Most important is to make a backup of message datafiles, profile data and uploaded images (if appropriate on your site).
Then setup a testforum using the newest release. Get it going first.
Then use either use commandline to copy messages into the new folder location or use the bulkarchive tool to move datafiles over. However, image files and profiles must be manually moved into new default location.
Regarding the out-of-memory error:
All depends on the size of your archive and the server limitations (nothing on earth is unlimited). You may need to split huge archives into smaller chunks. But that's only a very vague hint. Without more details one can't provide precise recommendation.
PS: would be nice to know what the URL of your forum is