Is reencoding a YUV 4:2:0 video in a 4:2:0 colour space lossless?
Probably no loss, but there is more to it than just YUV420. You can need to make sure the same ITU-R Recommendation is used (BT.601, BT.708 or maybe even 2020) In addition make sure the same color ranges is used (0-255 vs 16-235)