disk0 in lower bay, SSD upgrade

Mac Mini MD389 Late 2012, came with 2 x 1T HDD. OS is installed on disk0 (HDD1 in lower bay), other stuff - almost empty drive disk1 (HDD2 in upper bay).

I want to change an SSD and install OS on it to make it faster, but instructions said the easiest way to do it, to avoid the remove of the logic-board, power and etc, is to just add/replace the new SSD to the upper bay.

In doing so, am I only changing disk1 (HDD2) from spinning to solid. Can I setup booting device order and rename (swap) disk0/disk1, making disk0 to be SSD and install OS on it?

Or I have to pull everything out, and make sure SSD is positioned in lower bay to make it disk0.