To have Eisenhower run as a Democrat, you probably need to give him adequate cause. I am sure that there are Eisenhower experts on the board who will dispute what I am arguing, and I will concede that I am no expert. But, from the little I know of Eisenhower, he was, ideologically speaking, always a Dewey Republican. While he may not seem like a Republican in comparison to the Goldwater wing that later took over the party, he was nonetheless a Republican. Perhaps I am wrong, but from what I know, he did not chose his political affiliation at random. I am not sure to what extent his publicly declared lack of interest in elective politics before 1952 was genuine, but the fact that he entered does not mean he was being entirely facetious before that time. After all, a major reason that he entered that campaign was to block Robert Taft's Presidential aspirations. Had Taft died of cancer a couple of years earlier, there is at least some chance General Eisenhower never would have become President Eisenhower. Since Dewey was of Eisenhower's political persuasion, it's difficult to see Eisenhower running against him.
So either you need to change Eisenhower's ideology so he's on less politically friendly terms with Dewey, which may be difficult, or you need another kind of divergence, one which I do not know a precise divergence to accomplish. In short, I think you need to make Robert Taft the 1948 Republican nominee. If that happens, and it looks like Truman is a losing cause as it did historically, Eisenhower might be more open to calls for a draft on the opposite side of the political aisle. Eisenhower might decide to run as a Democrat if that's the only way to prevent a Taft Presidency. Of course, the plausibility of that depends upon how great Eisenhower's antipathy for the possibility of that eventuality actually was.
The question then becomes, how do you achieve a Taft nomination in 1948? That might be prohibitively difficult.