PDFTron is now Apryse. Same great products, new name.
You can purchase the AMS for your license at any time, but the AMS expiry date will be backdated to the time of the original license purchase.
For example, if you purchased your license in July, the AMS will also expire in July the following year, regardless of when it is purchased. Thus, it is more beneficial to purchase the AMS with your license and to renew it on time to be able to take full advantage of the subscription, when you need it.