I wouldn't use the 10K bulbs & also I would probably get some new bulbs to replace all the existing ones with 6500k.
Since you mentioned it's an old fixture I'm going to assume it's been used for several years.
I'm not sure if the bulbs the seller includes with the fixture will have any coating left to grow plants.
Every 10-12 months, you swap out the bulbs.
Most of my 2 coralife fixtures have died from overheating issues. I don't think I'll be going with this brand again due to what I've experienced. Even with the built in cooling fans, it didn't seem to make much of a difference.
Why wouldnt you use 10k bulbs?
But yes if the fixture is old. I would probably replace the bulbs.