The grace period for free upgrades as stated here:
https://support.presonus.com/hc/en-us/articles/360004040952-Is-there-a-Grace-Period-for-those-who-recently-purchased-Studio-One-3-
While "Updates" (example: going from version 3.5.2 to 3.5.6) are free. "Upgrades" (example: going from Version 3 to Version 4) are a Paid Product because the software engine has been completely rebuilt and is full of many new amazing features. Here is a link to what is new in version 4:
https://www.presonus.com/products/Studio-One/whats-new
If you would like to upgrade to our latest and greatest, Studio One 4, please visit your favorite PreSonus dealer or shop with us directly at :
https://shop.presonus.com/products/software/Studio-One-4-prods/Upgrades-Crossgrades-S14
Your current version of Studio One 3, is a fantastic product and we will continue to support it for years to come. It is still an extremely viable DAW for all sorts of music creation, mixing, and mastering!