There is no upgrade path from Halion Sonic 7. If you want the Halion Sonic Collection or the full Halion 7 you need to buy the full version outright. Halion Sonic 3 was a different product and the equivalent of the current Halion Sonic 7 Collection. The entire lot was renamed when 7 came out so its a bit confusing, but basically Halion Sonic 7 is by itself, there is no upgrade available from it.
Just like Groove Agent SE, there is NO upgrade path to the full version from the ‘freebies’ included with Cubase…
EDIT - While I was replying to this, someone in the Cubase forum just explained this exact thing to another user: Halion 7/Extreme 13 PRO - #5 by Martin90