If its an academic copy, the box should say something like Academic version not for resale. There should by a part number (I think it starts with M) by the barcode on the box do a web search for that code and you should see straight away what type of version it is.
With regards to your question about upgrading, the license key wont be (or at least they werent) compatible with the upgrade license key.
Can you remember what the discount was?
This is just going on memory, but recently (up till the new release of FCS), I dont think there was much discount on FCS through the HE Store certainly nothing like as much as there is now or previously. Because the pricing was so high through my HE Store, I thought perhaps that they had done away with a separate academic version if they hadnt it was economically much better sense to buy a full version and be able to upgrade, rather than get a marginally cheaper version that you cant upgrade. My memory may be playing tricks, but if this might explain the product you have.
Ive also checked the HE Store now and did a dummy order for FCS, selling for £250 the part number is MB642Z/A. Just done a Web search and that part number is listed by resellers as a full retail version.
Dont think youve anything to worry about!