SQL & PL/SQL

HI,
I have a table with 400milion records in oracle 10g R2
now i have 250 cloumns but need to check only for 3 columns

so the problem is each of these columns contain multiple email addresses seperated by semicolon.
currently the size of all the three columns is varchar2(2000).
so i need to write a pl/sql where i can find out which all rows shall be affected and what will be the total count when the domain name in the email address is replaced.

for eg>

suppose that one column has the following string
john@gmail.com;keity@rediff.com;sam@yahoo.com;peggy@aol.com;alex@yahoo.com

i need to find teh length of the string and what happens when the domain name for yahoo.com is changed to ttyyahoo.com

ie.
i neeed to have the count of the following.

john@gmail.com;keity@rediff.com;sam@ttyyahoo.com;peggy@aol.com;alex@ttyahoo.com


i dont want the other email addresses to change except for the yahoo domain name and i also dont want any changes to be done in the table. I cannot make a new table either nor do i have any permission to update the table

any pl/sql proc we can write here???