Dude if you don't know don't answer so definitively.
First of all, if people buying through riot can play with people who bought through uplay (they can, I looked this up when someone else asked, guessing same applies for steam), then odds are very good that all that matters is that are both on the pc platform. Second, this offer has the game for free, so even if it doesn't roll over, which I heavily believe it would, all it costs you is the time took to redeem the offer through riot, I would even venture a guess that you could then get rid of riot entirely if you wanted to.
The reason I am this sure, the data is saved on the server, not locally, when I played on Xbox my progression was the same whether I was on my brother's or my own, add to that you see your stats if you login to the game site on the internet.