They change the actors every season because Power Rangers in America follows the same formula that Sentai in Japan does.
If you look back to season 2 and 3 of Mighty Morphin Power Rangers (before Power Rangers Zeo, wich was season 4) you'll notice that in those two seasons you'll NEVER see the power rangers in the same scene as the monster.
The reason for that is because there is no Sentai footage to use and making american footage is expensive.
In Season 3 they started to use some more sentai by using the Alien Rangers story line and you could again have scenes where the rangers fought the monsters.
This is the primary reason that you have a new series every year.
Why they change the actual actors, honestly i dont know. Why would the same people need to change powers every year? It brings back the problem they had with the first 3 seasons.
Its better this way honestly. Though it is corny when you're supposed to have them being students in a California town, and the zords rampage through a japanese city.
Jason David Frank the actor that played Tommy is in fact coming back for Dino Thunder. Since he gave up his powers at the end of Power Rangers Zeo (or was it turbo? i forget) he's become a paleontologist and developed his OWN zords and morphers.
2 years ago, he and all the other former red rangers, including Austin St John made an appearance in a very badly directed and cut episode of Wild Force called Forever Red. Despite the continuinty problems with the episode it was really fun to watch.
Tommy, being the original 6th ranger, and longest running ranger across all seasons has always been the most popular power ranger with fans across the board. Thats why Doug Sloan decided to bring him back.
If anyone has any questions about Power Rangers and its 12 or so seasons i'm pretty knowledgable in it. Feel free to ask me anything.
And yes The original yellow ranger died a few years back.
Many actors from PR have gone on to have small acting careers.
Ranging from infomercials, makeup commercials, modeling, movies, and cellphone commercials.
Typcally you can find such info at
www.rangerboard.com in the PR sightings board.