[Colin F]

I would:

Backup your database and files.
Set up a testinstallation on your server or on your localhost using this backup you just made.
Go through the upgrade process on your testboard

Here you have two options:
1. Close your real board, pull another backup and upgrade the board
Install all the modifications you want

or

2. Try installing all your modifications and check if everything works. Make sure you note down all the database changes that are made.
Upload the (modified) vB3 files to your webserver and perform the upgrade like described in 1. Then, perform all the required actions to the database, test it to make sure everything works and open it.