New growth won't change because of styling on old hair, that would just be something that has changed inside your body. However, using heat tools that much on your hair, even with minimal heat, will cause some damage and as a result can break the curl a bit and cause frizzies. Maybe switch up your usual styling. Alternate curly days or even weeks with straight ones. Use the flat iron one week, and heat free large rollers the next.

Just remember the longer your hair is, the older the hair is as it goes down. That's a lot of days of heat to one area of hair
Originally Posted by BeckaMeadow
I completely agree with all of this. The heat styling will definitely cause damage but if you get regular trims to get rid of the split ends and, as the poster above stated, alternate between straightening and leaving it curly, it will improve the look the of your curls and make them perk up again.