I will soon be putting an SSD as my boot drive in a MacPro. I have ordered Final Cut Studio upgrade. My question is this: "Can I delete my original Final Cut Studio 2 for the purpose of less to clone to the new SSD and then install the upgrade? or should I just leave Final Cut Studio 2 as is, and after cloning to the SSD put the upgrade on? (Does the upgrade need Final Cut Studio 2 to work properly?)

You don't need FCS2 installed but you will need your original serial numbers ready for the fresh install of FCS3. If FCS2 was an upgrade as well then you'll need the numbers from your first Final Cut purchase.

Not too long ago I upgraded to FCS 3 on my brand new MBP. I only needed my serial numbers from my last upgrade, FCS1. I didn't have any previous version on this computer.

I've used FCP since the 1st version. After the first upgrade, I've never had to go all the way back to my first full purchase, FCP1, to upgrade.

Depends on the version you first bought. Upgrades have different codes. Final Cut Studio 1 was the first in a line of packages so it might have been different. I know that when I did a fresh install of FCS2 on a reformatted computer that I needed the FCS1 numbers before the install would go ahead.

You only need to enter the serial number from the FCS 2 upgrade to upgrade to FCS 3. You don't have to enter FCS2 upgrade and then FCS1 serial numbers, at least for my installation from the FCS 2 upgrade.

I thought that if you install an UPGRADE version, you have to put in the serial number of a normal RETAIL version too, and not a serial from another upgrade version... But I could be wrong.

Btw, when I upgraded, I used FCS Remover (a free tool, available online) to uninstall everything from Final Cut Studio 2, and then I performed the upgrade as a fresh install. Because that was what I wanted - a fresh install.