I have tried the "0.44 testing" version with two save games, one from 0.38.2. and one from 0.43 (both Win 64). Both were converted without any problems.
I do like the new UI in general, especially the hunger bar and the daytime display. However, I don't like the colors of the oxygen, health and hunger bars. I would prefer them to be displayed in default hud color (orange) and have them switch to flashing red or something similar if in critical condition.