There are some characters in Chinese which give me severe headaches, especially those that have the same pronunciation and mean more or less the same thing.
I have learnt three characters which are all pronounced shēng: 升, 陞 and 昇. Is there any significant difference between these characters in modern usage? I'm talking about traditional Chinese here, I don't think 陞 and 昇 are used at all in simplified Chinese.
The answer to this one turned out to be fairly straightforward (assuming the website I found is accurate). The following is quoted from this website:
升 ① 容積的法定單位（市升與之相同），1升等於1000毫升。 ② 量糧食的器具，容量爲斗的十分之一。 ③ 向上，高起：升旗。 ④ 提高：升級。 昇 同「升」③④。 陞 同「升」④。 （《新華字典》第436頁左列「升」字）
說明：「升」「昇」「陞」三字在「登」的意義上相通，以「升」為正體。「昇」本義為太陽上升，例如「旭日東昇」，用「旭日東升」也正確，需要注意的是，「歌舞昇平」似用「昇」更正式。在一些人名上，如淸代寫《長生殿》的洪昇，宋代發明活字印刷術的畢昇。「陞」，古代「升」的寫法，可用於當今書法。 舉例： 昇──昇華，昇天，歌舞昇平，高昇。 升──毫升，升高，升堂，升班，直升機，回升，調升，提升。 陞──步步高陞，指日高陞。
So, in short, it's safe to use 升 in most cases (except in some names and fixed expressions).
What the two entries above said is true.
An addition, however, is that the simplest version, 升, is the only one of the three that can be used as a unit of measurement of liquid.
I've also seen both 升 and 昇 used in mens' names a lot, but 陞, not so much.
