Questions about 4k upgrade

I currently have Cloud DVR service and these boxes: XG2v2-P and two XiD-P.

(1) Can I upgrade to 4k boxes at the same price? (1a) Does upgrading involve replacing all three boxes or just the XG2V2-P? (2) Will the new boxes connect to the TVs with the same HDMI cables? (3) Does it matter that the TVs are not 4k? (4) Does it matter that I have my own Internet/ voice cable modem and not an Xfinity cable modem?

Also, I've experienced a long-term issue of voice and picture not being in sync.  Might upgrading solve that problem?

Thanks.