You can use the Open View dialog to search all forms and procedures for instances of a variable name.
There is no way to do a search and replace across all forms and procedures. Rather, I should say that there is no user interface for doing that. It would definitely be possible to write a program to do that. But you would want to make absolutely sure that you were only replacing what you thought you were replacing -- you could seriously wreck your database in a hurry if you weren't careful. Which is why I haven't implemented a user interface for doing this, at least so far.