Hangetsu and Seisan were once the same kata, but are now very different 

Good comparison, but I think a better one would have been Shotokan and Shito-ryu. Both Funakoshi and Mabuni were students of Anko Itosu so there a common lineage there. Likewise Mabuni also shared an instructor with Chojun Miyagi. But since Shotokan and Shito-ryu share that lineage it would nice to see their differences. Did Funakoshi change the kata? Or did Mabuni favor Higaonna's version over Itosu's? Did both Itosu and Higaonna practice the same version and Funakoshi made changes of his own?

I did not realize ?.....I regularly practice seisan, and recently learned hangetsu.......I will be much more focused when I practice these from now on Nice insight

kata is always changing, its part of the evolution and understanding of the movements and its intended aplications, in the case of seisan/sesan/hangetsu those changes are obvious and sometimes its even dificult to recognise the kata itself, even within the same style there are several variations of the kata, Shorin ryu for instance uses both matsumura and tomari versions of seisan and they look very diferent.

Comparison isn't correcta at all. Author of this video compilation should understand that on Okinawa there are THREE mainstreams of karate, containg Seisan kata. You see on video at right side Goju-ryu Seisan. There are also Uechi-ryu seisan. Both are far away from Shorin-ryu Seisan and nothing to do with Hangetsu. In group of schools named Shorin-ryu style there are also few versions but they are common in their source. And one of Shorin-ryu Seisans became a forerunner of Shotokan Hangetsu (Seisan) and Wado-ryu Seishan.

This is not the only kata that has been greatly changed over the years, Sanchin no kata as well, etc. Evolution?. Rather transformation according to the good understanding of the senseis. The important thing is that the old katas, their meaning, their bases and the bunkai are not lost. How enriching it would be for senseis of different styles to teach the keys of kata to other senseis, instead of being closed to any new perspective.

i think you might be right. thank you

Barely similar moves, the changes made from Seisan were a re-write not a mod!

a better link is these forms. ru-vid.com?search_query=fujian+lohan+quan

Another video made by someone who knows nothing. Shotokan Hangetsu is based on Matsumura Seisan, not Higaonna Seisan as depicted. Do some research.