Prior to 1956, popular Romanization of Chinese was based on Wade-Giles. After 1956, Mandarin Chinese Romanization was based on pinyin.
In pinyin, the letter “i” as a phonetic is not found. In the pinyin dictionary, the last “H” entry is huo`, and the next entry is “Ji”. All traditional “I” sounds used by Wade-Giles were grouped under “Yi”. So according to pinyin, all “i” (ee as in tea, see, sea) sounds are listed and spelled out as “yi” (yi (1), clothing; yi (2), to move; yi (3) already; yi (4) meaning, translate, easy, different, righteousness).
I Ching is Wade-Giles. In pinyin, it should be “Yi Jing”.