A roster patch is usually only the players.dbf, that is put in your save game folder (/My Documents/NBA Live 06/saves/*name of savegame*/). To completely replace the roster, just replace the players.dbf with the new one; but I don't think there's an easy way of doing so that you keep the stuff that you want. The only way I see doing that is to open up the new players.dbf (with a program like
this) and make the same changes in there that you've done earlier in your dynasty.
Here's an explenation on what the different things meen in the player.dfb (note that it's for 2005, 06 might be a little different)
http://www.nba-live.com/nba2005/dbf_players.php