all software vendors do this, they are not the only ones. a lifetime upgrade applys to the version you have, if they decided to add something different to the program that was not there before, then it is no longer the same piece of software.
your lifetime upgrade applies to patches, not necesarily code rewrites. perfect example is MS Windows. how many purchased say Win95, filled out the registration card, since it said you are entitled to free upgrades. yet out comes Win98, and I did not get it for free! it was a new product -yet pretty much the same code. but I now have to pay for it.
thats the life of a software title. free upgrades for the life of the product, but when the life of the product gets core changes, its not the same as it used to be. therfore it is a new product, and if ya want it, you must once again, pay for it.
this happened to me with NTI CD-Maker 3.51, free upgrades, but only up til v3.91.
Once version 4 came out, no longer get free upgrades.
If you want to sue them, be sure you read the user agreement, see if it states, they can change the policy anytime they want. if so, you agreed to it, and you can not sue them.