I think you're a bit confused by the process.
To upgrade the machine you have MC19 on, you must download and install MC20. The license doesn't do that, and MC19 will never do it. You have to. Download the latest version here and install it:
https://yabb.jriver.com/interact/index.php?topic=99073.0Then, you can use the MC20 registration code you got
inside of MC20 on that machine, and it will install the license. You
cannot use a MC20 license in MC19 to make it "upgrade you". In fact, since you probably used up a few of your restores already, you were probably mailed a bunch of MJR files by JRiver. Just get one of those out of your email, and double-click it (again,
after you've installed MC20 on that machine and opened it) and it will install the license there for you (without requiring another restore).
Don't worry about using up all of your Restores. They can be reset. It is
not a download limit.
Please read through the article I linked above carefully. It makes the process pretty clear.