I'm not a professional, but it sounds like it shouldn't be too complicated to do, as it's not like your changing extreme levels. I could foresee difficulties if they were trying to lift your color from dark brown/black, which what I was worried about
So maybe you want to have an all over color perhaps of what your highlights are to match Avrils? Sounds like what you have now is a jen aniston color mix going on (i love that) and just want to lift the caramel?
I hate it too, when they 'insist'. If there was risk involved with lifting to platinum, why don't they just explain their reasons, and then still leave it up to you as to what you'd like to do?
my friend took pics to her colorist, wanting to go from her Jen Lopez shade to a Jen Aniston shade. He insisted on a just doing a bunch of foils. It sounded like after all the time spent, he only used a 10volume, and her hair looked pretty much the same $130 later....she was still brunette!! And then he tells her to come back next week whenever he has another appt avail, to try it again. When they do this(which sounds like he messed up with the wrong volume), and you're not satisfied, do they expect another tip at the next appt??? So yes......why do they have make things so complicated and frustrating?