The Mach3 version, also called the Addons for Mill that you paid $50 for are good forever, for all new versions. These are the wizards that run under Mach3.
The new Mill Wizards, that are a standalone program will need a new license, the introductory price is $50 the list price in about a month will be $75.
There are other differences. The new license is locked to a PC, you can run it only on one machine. You can run it in demo mode on any PC to create a job file, and view the toolpath, then take that job file to the machine with the license and generate the code. Bug fix versions will not need a new license, but if we come out with a major new version it will need some kind of license upgrade, but that is not fixed yet.
Think of Microsoft- you bought XP, got a couple service packs for no upgrade cost, but if you want Win7 you need to upgrade