To float or not to float

Hi
Have a query and are looking for guidance? I have a wooden floor which is sound in level but a bit ugly, I plan to overlay this with a tongue and groove timber flooring, planning to remove skirtings, to not worry about raising the height as overlaying will result in, but my big question is do i Glue , nail, screw or hopefully will be advised to "float" on the existing floor, considering a layer of insulation, again advice would be much appreciated.

Thanks
Russell

I reckon it might slide a bit if wood on wood, but a bit of laminate underlay could hold that, possibly spray-glued down?

The existing floor, is it a laminate or engineered wood floor, re-finished floorboards etc????