It certainly isn't impossible to change a kata - just about every kata out there has been changed from its original version. They have been combined, shortened, and otherwise modified by numerous masters. A single kata, like the very common seisan, has been changed to be practically unrecognizable from it Chinese roots. Only a couple of signature moves provide a link to the "original."
Sanchin was introduced to Okinawa by Higaonna (most likely in a different form than his Chinese teacher taught him) and passed down to Miyagi who made some major fundamental changes, who then passed it down to Shimabuku who shortened it a bit.
Itosu played around with kata to come up with the Pinans and Funakoshi made big changes to traditional Okinawan kata to suit them to be taught to Japanese school children. So much for TMA kata being static and written in stone as some uninformed may claim.
All those named above were recognized by their peers in the tightknit karate community as being on the highest level of knowledge and skill - their "right" to change and create was unquestioned, perhaps even expected. Unfortunately, this type of built-in quality control no longer exists in much of today's karate environment (I hesitate to use the term "community" these days in the West).
There is still change going on - some good, some bad, depending on whose doing it and why. I hope that in another 50 years the art will still retain most of its traditional integrity and not be totally stripped for sport or entertainment value.
