My strong suggestion is to script all of the stored procedures (including their permissions) using SQL Enterprise Mangler, and save two copies of the script. Edit one of the scripts to suit your fancy, then play it into your database. Test, test, test, then test some more.
When you discover what you mangled, play in the pristine script and start over!
Are the procedures stored in files or do they solely exist in sqlserver? If the latter, you have perhaps a challenge. I'm not sure if this will work: select the inside from syscomments (column text), then change what you want. You could commit the change to the syscomments table but you'll have to reconfigure sql server to allow this and I am not sure if that's a good idea. Besides, you'll have to think about having to let sql server know the procedure changed so it'll recompile (in order to make sure it'll use the latest version). Instead you could try to have the contents of the procedure executed using sp_executesql, don't forget the drop and create commands though.
I would not recommend either one since you'll have some issues to consider prior actually doing the changes.