mixing brands

The secret to making cable selection easy is to properly match your components. Get that right, and its far more likely that you will be able to use one brand. If you need to use cables to fix problems that you didn't take care of by selecting the right components, its just endless trial and error.

In your situation, if you put your system together right, and just can't afford to do all of your cables at once, by all means mix them. If you like the sound of the new cables better than the old ones, your system should sound better with each new cable you swap out.

I don't remember the brands I use at all. Definitely on speakers I have Kimber 4tc unterminated on both ends.
Interconnects are mostly pro-audio XLR Mogami gold/silver

I have a lot of experience with Nordost as a system and combined with other brands. (Although the last was before the "2" of the Norse and Valhalla range came out)

If your plan is to change to another brand I would certainly change the speaker cables first.

In most or almost all cases I had Nordost ic's worked fine with other brands of speaker cables but combining their speaker cables with other brands of interconnects did not work fine for me in all but a few cases.