Indeed, reuersio is what you want to look up. Here's my first-order approximation of why this is so — I trust that it is not too inaccurate!
The classical Latin alphabet had only the letters
A, B, C, D, E, F, G, H, I, K, L, M, N, O, P, Q, R, S, T, V, X, Y, Z,
where I represented both /i/ and /j/ and V represented both /u/ and /w/; by the Middle Ages, the Latin alphabet had also acquired more-or-less the now-familiar lower-case forms
a, b, c, d, e, f, g, h, i, k, l, m, n, o, p, q, r, s, t, u, x, y, z.
However, j and v only appeared still later, as allographs of i and u, respectively, and it wasn't until after the Renaissance that i and j systematically represented /i/ and (what had at least been) /j/, respectively (e.g., cujus for cuius), and u and v systematically represented /u/ and (what had at least been) /w/ (e.g., vos for uos).
At any rate, since the Latin orthography of classical antiquity only knew I and V (which originally gave rise to the lower case forms i and u), many modern editions and references use only upper-case I with lower case i and upper-case V with lower-case u, (I presume) in order to adhere as much as possible to the original orthography whilst allowing for the normal use of upper- and lower-case letters.