Importing a board 
Warning:
Make sure you have a 100% completed and working backup of your currently working forums (files and databases). If needed, you can revert back to your previously working version and start over again

From your Admin Control Panel (admincp/ by default), select Import > > Import.

Select the system you wish to import from the list and click on the [Begin Import] button to confirm your choice and start the import procedure.

001 - Check and Update DatabaseThis module will check and alter the tables in the databases as well as the connections. After clicking on the [Start Module] button it check the database for the tables it expects to find, you should see a green list of you source database tables, and tables listed in red are expected by the importer and missing, you import may still run as not all tables are used during an import, though if critical tables (i.e. the user table) are missing then this will break an import.

ImpEx will Alter the tables in the vB database (target) to include import id numbers. (This is needed during the import process for maintaining references between the tables during an import.)
If you have large tables (i.e. lots of posts) this can take some time. They will also be left after the import if you need to link back to the original vBulletin userid.
002 - Associate usersYou only need to run this module if you have users existing in the vBulletin database that you wish to merge with users in the source database during the import that have different email addresses. If you are doing a clean import then skip this module.
Go through each module one by one (starting at 001) and let each page load 100% before clicking on any links, do not press the [back] button and don't move away from the screen when it is running. Read carefully what is on each screen and write down any error in full before contacting support.

To start the import click on the [Start Module] button and when that module has been completed, the statistics will update and the name of the button will change.
Note:
klancy 17th May 2010, 04:58am
Presently, 3.7 shows as the highest version. I used to have a v3.85 board and have upgraded to 4.x without any problem. If you are upgrading to v4.x, just select the "3.5-3.7" option and you'll be fine. This would be a good thing to keep an eye on since it could confuse some users.