Necking it down is as simple as running the 6.5x284 brass into a 6mm-284 die, and you're good.
If you have 284 brass, and neck it down to 6mm, the necks could become too thick. If you take 6.5x284 and neck it down (recommended), it will have it's own neck thickness different from the 284, and most likely a different brass I would select 1 brand of brass to work with when necking down, and stick with it. Otherwise you will have different internal case capacities and different neck thicknesses between different brands. If you size down 284(7mm) brass, you will want to do it in 2 steps- size to 6.5mm then to 6mm. Necking down 7mm to 6mm in one step is a .041", which is a lot in one sizing. The most I will go is .030" in one step. The easiest is to go 6.5x284 to 6mmx284.