Regardless of which version you are changing to/from you will always need to run all your tests on your scripts again. You do have a complete set of tests, don't you? If you don't then now might be a good time to write them.
You should also carry out a sequence of parallel running with both versions - there should be no problem with having two versions installed provided your #! lines and/or your PATHs are correct.
Why so old versions? 5.8.4 is at least seven years old and there is a lot of really cool stuff in the latest releases. Many people stick with 5.8.8, and that is pretty stable, but to go for some abritary maintenance release seems strange.