If one has a seminal emission at night and wakes up before sunrise to shower change clothes etc. does it make it so that he is tahor? Or must he wait a full day until sundown in order to do this?
If the point is wat is "ובא השמש וטהר", the answer seems obvious. About shmirah of shomeret yom keneged yom, One Tana, Rabbi yossi thinks that night may be shimur. But about Haarev shemesh Gemara Brachot 2a&b said explicitely that the happening of the night is the criterion. Before night the cleanness candidate is called Tevul yom and cannot eat Teruma i he is a Cohen and cannot eat meat or backeries of Korbanot:
But how do you know that these words 'and the sun sets' mean the setting of the sun, and this 'we-taher' means that the day clears away?(Berachot 2a)
But as explained me @msh10, the question is about Takanat Ezra, an ordinance concerning men after sperm emission. So, it should be noted that Tvilat Ezra has no rule of Tevul yom. Immediately after immersion, "Takanat Ezra's speaking", this man is clean. Even more, this the rule is the same if his "emission" appened at morning, and the mikveh or pouring 9 kabim (see later) immediately after. Changing clothes is not a part of th Ezra's ordinance, but when sperm is not dried he is considered as a soiling liquid like urine.
The kula of 9 kabim is to pouring water divided at the most in three times on the Baal Keri. I have no direct information if a simple shower is good, but you can read here that some Rabanim allows this practice with shower. One instant after he is entiteled for tefila, torah and all holly activities. BTW the tevila is annulled nowadays: "בטלוה לטבילותא". For more general explanations concerning Tevilat Ezra, see this post