There are profession councilors in Mos Eisley, Theed, and Coronet just outside of the starports - Twi'lek females in white. You may respec the first time for free, after that it costs credits.
You may go from any combat class to any combat class (jedi, bounty hunter, smuggler, medic, commando, officer, spy) without losing level; however, you will lose any XP gained for the next level. In other words, if you midway between 20 and 21, you would revert to 0 xp of level 20.
You may respec from any trader to any trader with no loss of level.
Any time you go from trader to combat class, you will start at ground zero and start from scratch - the same whe you go from any combat class to trader.
All classes start from scratch if the respec to entertainer; entertainers start from scratch changing to any other profession.