In November last year I registered a copy of Studio One Artist V1.5 with Presonus. I had the CD for a few years but I never used the software before. At the time of registering there was already Studio One Artist V4.0 on the market. After I registered with the product Key of my V1.5, Presonus did automatically upgrade my Artist Version to 4.0 at no cost.
I think, they still upgrade you to the current version of Artist.