Olga_L,
The difference between improve and improve on/upon:
improve (trans)....to make better
improve upon or improve on.....to make better than; to advance beyond
However, overall face-to-face communication can be seriously improved upon. =
However, we can seriously advance overall face-to-face communication beyond the email policy changes we made two years ago.
Basically, Eliot is correct. "Improve upon" is used to make a comparison with the ongoing project mentioned in the first sentence.
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However, overall face-to-face communication can be seriously improved....This sentence could be construed as a generalization not dependent on the first sentence.