Lossless conversion of yuv420p10le to yuv420p
No this is not possible. Looking at this theoretically disregarding the codec you can not take away information and call it lossless, thats already a terminology problem.

Applying a new color space without transcoding is also not possible
It's not so easy to say whether this is even possible with certain codecs other than h264, it really depends on how you define certain terminology.

Regarding h264 its simply not possible in the way the codec works, color information is essential in how the codec encodes frames as its based on motion estimation and the color information between frames is what defines the in between frames that don't hold the information of a whole frame but only the differences between keyframes.
So just applying different color values to each existing pixel is not really possible. It would result in major artifacts.