How can I convert collation tabel, field to utf8_general_ci

[fastcom]

My forum upgrade to Vbulletin V4.02, in this version it's do not support search unicode and I want to convert all table and field collation from latin1_swedish_ci to utf8_general_ci. I user this code

http://www.phpwact.org/php/i18n/utf-8/mysql

It's convert all collation table to utf8_general_ci but it's can not convert any collation field to utf8_general_ci. When I try use SQL query to alter the field of table it have some error
example:

ALTER_TABLE 'searchcore_text' CHANGE 'keywordtext' 'keywordtext' MEDIUMTEXT CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL

# MySQL said: Documentation
# 1283 - Column 'keywordtext' cannot be part of FULLTEXT index

How can I fix it, I wanna to search unicode to my forum, CMS and blog.

Thank a lot
p/s: This forum(vbulletin.org) can't save query string in tag code => bad request