Yes, premade sims that you download can also have 'left-over' buffs with any previous traits that may have been removed by you on first downloading them or by the original maker etc. This is why in challenges/competitions brand new cas made sims are required. If the sim has never been played and so is a newly made sim in CAS and then you download it... well, I would think that changing the traits wouldn't matter but changing the aspiration trait might.
Some traits don't really seem to leave behind anything much but there are hidden attributes associated with various aspirations traits too so changing those can change how your sim reacts or skills up etc. You can see this in some abilities. For example some sims can knock out Best Seller after Best Seller but others, with seemingly the same traits, really struggle. Much like collecting and fishing, changing the aspiration around even in a new sim while in CAS can have different effects on game play.
It was all explained to me in technical terms, but seeing as I am not particularly technical I have really butchered this reply and explanation. Sorry 'bout that.