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I'm surprised by this question because there is extensive halachic decisions instructing us as to how we may add water to a pot of cholent, whether on a blech or in a crock pot. Rabbi Moshe Feinstein, zt'l, for example, wrote a decision permitting the adding of water that was kept hot, but not as hot as the contents of the cholent pot, to prevent a cholent from drying out. Iggros Moshe O.C. IV:74:Bishul #1. Orchos Shabbos 1:1:17 cites others who are machmir about this. The method there, I believe, involved taking water directly from the kli rishon to the cholent pot. However, Rav Shlomo Zalman Auerbach, zt'l, even allowed dispensing hot water into a clean and dry cup (kli sheini) to pour into the cholent. Shemiras Shabbos 1:16. These sources and others are cited on Star-K's website. Based on these sources, it would seem to me that you can't add water without removing the lid, so removing the lid on Shabbos must be permitted. Consistent with that conclusion, I remember learning a shiur by Rabbi Gedaliah Anemer, zt'l, who was partial to a thicker cholent, where he said it was permitted to lift the lid of the pot to allow some steam to escape. In all of the above cases, the lid is not removed until the cholent is fully cooked.
However, apparently there are sources who would consider the simple act of lifting the pot cover (while the pot was still on the flame) to be maygis (stirring). See Bartenura (Shabbos Ch. 7, Mishna 2, s.v. haofeh), Shulchan Aruch (O.C. 254, 4 & 257, 4), Mishna Berura (254, 23 7 Biur Halacha 257 s.v. gorem), Mekor Chaim (118, 18), Shvisas HaShabbos (Mevashel 26, 81), Ketzos Hashulchan (124), Shu”t Igros Moshe (O.C. vol. 4, 74, Bishul 10), Chut Shani (Shabbos vol. 2, pg. 197), Shu”t Titz Eliezer, (vol. 7, 15), Shu”t Teshuvos V’Hanhagos (vol. 1, 207, 3), and Shmiras Shabbos K’Hilchaso (new print Ch. 1, 41). These sources are cited in an on-line article by Rabbi Yehuda Spitz at Yeshivat Ohr Sameach's webpage. It is not clear to me whether these poskim are speaking only about before the cholent is fully-cooked, or whether they would still forbid taking the lid off after the cholent was fully cooked but before excess liquid had evaporated.
These opinions to me seem to be contradictory. Consult with your local Orthodox rabbi for guidance.