I hope they get new ones.
With all due respect, guys, I don't want existing straight companions to suddenly turn bi at once and hit on my character. That would be wierd.
As far as I can tell, you have to start any romance.
Also, technically speaking, they wouldn't "turn bi". Remember, your companion (story wise) only exists for "you". Like in DA2, it's not so much they "turn bi" as their orientation is changed to fit what you need.