I am changing my db so that certain fields can be longer. Right now they're "varchar(255)" can I just increase that to "varchar(500)" or are there special rules for dealing with this and I should make them "text"?
From the MySQL manual:
"The effective maximum length of a VARCHAR is subject to the maximum row size (65,535 bytes, which is shared among all columns) and the character set used."
So, yes, you can just extend your columns to be a maximum of 500 characters in length (though, you'll want to be sure your application logic doesn't expect the maximum length to be 255 characters).
Edit: see also cballou's note below about character sets.
Modifying to a longer varchar would be fine:
ALTER TABLE my_table MODIFY column_name VARCHAR(500);