The error message you're getting indicates you have a problem with a table in the database, and a repair of that table will probably take care of it. Check out the first link in my signature......
At worst, you can drop the search tables and rebuild them from scratch using a MOD (such as this one
But yes, you DO need to update. If nothing else, you can run the database update that is included in version 2.0.23, just prior to importing the data into phpBBv3. You may find that the MODs you are using are already included in v3; most of them that I use are there. But, two are not, so I'm staying with v2 until v3 gets these last two MODs I