As my hair grows, it seems to be less curly, more like 2b-c-ish looking. Some parts are actually straight. Will I lose my curls as my hair lengthens and heavies? I hope not. I have finally become comfortable with my curls and it seems like one day I may just have waves.

Has anyone experienced this? What can you offer for advice? Thanks.

Well, it depends on your hair growth pattern, among other factors. 
As for me, my hair was curlier when it was shorter.  I now have loose spiral curls most of the time.  It get curlier with the humidity though.
This is part of the journey!