As with anything to do with computers, the first thing you should always do is backup and save your work regularly.
This applies especially more so when you are about to make changes. This step simply duplicates your base as it currently exists and creates a backup for you to refer back to if you make a mistake in the upgrade process. It takes the pressure off, as you can't do anything to harm your precious database by having a copy backed up.
This also enables us to be able to jump in and help you out too if you really get stuck, but it's pretty straight forward.
Are you ready? Let's begin...
Hopefully that was fairly straight forward for you. Now you know how to backup your base by creating a duplicate, you can do this regularly so your data is always safe. It's recommended if you use your Merch by Amazon account daily to create a backup once a week. If you only use it a few times week, then once a month should be enough.
Ok, on to the next step - 2. Adding the Niches Table