With each new version there is a complete program that is fully operational. FTM can be added to your computer with out changing the version you currently have. There is on caveat, once you import your database into the new version it is no longer useable in the old version. There for make a copy of you old database before importing it in to the new version. While most of the time you will have no problems, why take the risk. Note: In the past couple of years, there have been true updates, but you access them from with in FTM.
Ancestry will add the additional months in your new purchase to the end of your current subscription. It may take some time (months) for it to appear in your account