Demi/semi permanent color on top of permanent??

My natural color is dark golden blonde (maybe level 6 or 7?) and I've been getting it colored with a permanent color for about a year. The dyed color is a 6 with golden copper base so it comes out to be a medium auburn color. I love it (though i'd be ok with a level lighter and more copper than gold).

Anyway, I'm wondering if I can use a semi or demi perm. color over the current color and on the regrowth. I'd want to keep the same color (med copper/auburn) but what will this look like on the regrowth? Since the dyed color is darker, if not the same level, as my natural color I wouldn't have to worry about the demi not lightening. But would the regrowth look different than the rest? I'd love to try redken shades eq but want to make sure it will work.

Thanks!!