Some additional info:
Upgrades to the latest version of MC are heavily discounted. They cost less than half of a new purchase price. They are really affordable, even if you only upgrade every 2 or 3 versions. JRiver will let you upgrade from any old version to the latest. So from 26 to 31. Or from 20 to 31. Or 30 to 31. It's all the same discounted price.
What I've found is that there are sometimes key things that get added or changed along the way that I really want. So it's easiest for me to just upgrade each cycle. You may feel differently.
Finally, every now and then, there's a major change that breaks some part of older versions. The most recent example is when JRiver had to change their long standing servers from one place to another. This broke cover art lookups. So, they decided to put in an emergency fix backwards for several versions. I think they went back 4 major versions, but don't quote me. If you have one of those versions, lookups work again with the fix. If you have a version older than that, the program still works, but cover art lookups do not.
In the 8 or 9 years I've been using MC, I can really only think of 2 or 3 things like this. All versions still play your music and videos. Some auxiliary functionality might be lost, but those changes are very rare.
Finally, JRiver is a small company that makes a best of class product. I like supporting them with the small amount of money I pay for each upgrade. This is partially a selfish decision. My purchases may help them stay in business and keep supporting my favorite media manager and player: MC.
Take care,
Brian.