Yeah I would say you should call up Apple and find out from them. If there is an upgrade, I'm sure they will tell you about it.

And in the event that you did download Final Cut off the internet, I suggest you just save up your money and buy FCS2 Academic. I've tried billions of times to get it from the internet, and I succeeded once, but then a month later my serial number wouldn't work anymore, and neither would any other serial numbers I found. Apple is really careful about making sure their products aren't stolen. So I just saved up my money and bought FCS2 and it saves a lot of the hassle.

Hey Blake

The usual solution to "my app won't update" issues is for the user to move the app back into its default install location. I don't know how many times I've had to trouble shoot this one and it almost always comes down to the same thing ... despite protestations to the contrary the app has usually been moved to the Desktop instead of a just shortcut to it having been created there!

Your situation is different. You have 5.0.4 and are now looking to upgrade to 5.1. Unfortunately, as Robert already noted, Apple won't sell you an older version/upgrade once a newer version is shipping, so the upgrade to FCS 2 is the only obvious way forward for you. FCS2 is a great upgrade!
The only other alternative is to look to retail outlets other than Apple and to see if they have and can sell you any old stock.

Best
Andy