11GR2: Need to change size of column

Hello,

We are on 11.2.0.4 database.    We have a table which has more than 30 million rows.    We need to change  length of column from   2 chars to 3 chars. .    Since this is a 24X7 system,   we do not have a downtime to do this update.  

Will using "Alter table modify column" impact  inserts/ update to table  ?    What are the best way to change the length to avoid downtime /  lock wait issues ?

Thanks

Ram