Yes, of course you can replace a string in all fields of a specific table in a specific databases by using the REPLACE function.
No, you can not easily do this on an entire database… you have to script this.
I have found a Brilliant script @BrillianSheep.com which I modified a little bit for readability…
Copy/Paste the script in a php file, replace parameters (strings to replace: yourDbUser, dbP4sswd and yourDbName and run it on your server!
Thx a lot!!