Are the players from a previous team, who's position has been taken over, just retired and no longer available?
Yes.
A small number of those players, who are deemed too good to perish, are released on the market as free agents. The determination, I believe, is made based on the player's position and salary.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."