Use ALTER INDEX to perform the following maintenance tasks for a Text index:

Rename the index.

Rebuild the index using different preferences. .

Resume a failed index operation (creation/optimization). .

Process DML in batch (synchronize). .

Optimize the index. .

Add stopwords to the index. .

check this link for more detail

http://lbd.epfl.ch/f/teaching/courses/oracle8i/inter.815/a67843/csql.htm#14189

I dont think u can add are change fields to indexed, best is drop and recreate again