OK,
Thanks for the further information. I would suggest that you use PhpMyAdmin for this job.
This will allow you to backup your data and then either alter the data or empty the tables.
PLEASE PLEASE PLEASE PLEASE PLEASE PLEASE make sure that you back up your data before you start, just in case you make a mistake while you are changing things around.
There are some functions within phpmyadmin that will empty tables and will also REMOVE tables from your database so make sure that you have copies BEFORE you start.
Just post back if you need any help and please remember to mark this thread resolved once it is finished - it helps other users.
Cheers,
Neil.