I would keep the 6000k and simply add the 2600k lights to the mix. More light usually can't hurt. If it's a choice between 6000k or 2600k lights for space or temperature control then opt for more 2600k obviously. Ideally, you'll want anywhere from 10-20% blue light to supplement the red lights when flowering.
Also, if you've got plenty of blue light on a flowering plant, the plant will need a bit more nitrogen then a typical flowering nutrient regimen.
No worries about slowly changing lights, you can do the add/switch complete at any time.