Chroma reconstruction 4:2:0 to 4:2:2 or to 4:4:4 is a key feature of Video Artifact. Some users do not understand how it is possible. It is like a magic.
I use warp sharpening in U and V with thin edge mask from Y. This way cleans edge pixels. There are another ways to do it, for example using blurring or anti-aliasing, but the VA method works perfectly with noised sources.
See the image below. It is a chroma channel reconstruction from Panasonic G7.
Warp sharping produces clean edges for aliased ones. The result is a smooth line after sawtooth. But image will look very strange if used for whole image, so edge masking is used to process only image edges and only in U and V. Visually the result looks like native 4:4:4 and can be used for green screening where it is important.
Another 4:4:4 reconstruction part is a motion denoiser that summarizes many frames after motion compensation. After 4:2:0 image (if stored in 4:4:4) we have many image fragment averaging, so flat parts will be restored, too. Similar way is used in astronomical photo to restore stars from very noised sources. Motion images give us a way to get sub-pixel details after motion interpolation. So the software also can restore complete destroyed details after video compression like next example:
Of course, if we shot two close colored pixels in 4:2:0, they will be complete blurred and there is no way to get them back. Only parts those can be restored are restored. So no magic is used. It is just a result of my 20 years knowledge in image processing. And you can use it with the Video Artifact editor. It is a filter RepairChroma8.
For best results different settings for U and V channels are required. Also pre-denoise gives better looking edges. This filter helps also to de-blur chroma from some Sony NEX cameras.