I am not sure. But I bought New Leaf from the eshop before the WA update and it keeps telling me that New Leaf WA is reccomended for me to buy from the shop so maybe you could do it that way but I don't know if New Leaf and New Leaf WA are seperate anymore.
It's effectively possible to install both the original New Leaf and the one with the Welcome amiibo update built-in on the same system. Since they're treated as separate games, each will hold their own save data, StreetPass data, Extra Data and Update data.
If you haven't purchased the game prior to the Welcome amiibo update, then this trick can't be done.
i got the game back in 2013. is there a link to how you can do this?
As I said before, the ACNL you downloaded back in 2013 and ACNL Welcome amiibo are treated as separate games. Therefore, you just need to go on eShop, search ACNL Welcome amiibo (the one with green leaf icon partially surrounded with colored dots on a white background) and purchase it. Doesn't get any more complicated than that.