I don't think the change in appearance was all that dramatic, although it can certainly seem so in that we don't have many pictures of Ueshiba in the 40s. Obviously, there weren't a lot of pictures being taken from his retreat to Iwama in 1942 until after the war ended and budo was allowed by the Occupation. He was 59 when he left for Iwama, and well into his sixties when the post war pictures start appearing.
a picture from the Noma Dojo selection, in 1936 (age 53). The quality isn't good, but if you have the English edition of "Budo", it's clear that his handlebar moustache is graying.
Then you have this picture
, one of the latest pre-war, pre-Iwama ones. He's still looking quite hale and bold, but his moustache is all white, and if I'm not mistaken we can see the beginnings of the beard.
of the early post-war pictures. Early 1950s, and Ueshiba's in his late 60s, but still looking pretty hale. But now he has the full beard. The appearance of the full beard, more than anything else, accounts for the big change, I think.
If you have Budo: Teachings of the Founder of Aikido, and/or the first volume of Saito Morihiro's Takemusu Aikido, I think the pictures show a clear and natural progression.