Glad to see another scan in the series. Good news is that we are translating it, bad news is it is going pretty slow. Any TL interested in helping to work on this, please PM me :)
This indeed isn't Belldandy, but Urd under a disguise. It seems that, Belldandy is temporarily turned into a kid, and to avoid Megumi, as well as Tamiya and Otaki finding out about it, Urd had taken her place by altering her appearance.
Keiichi even notices how bigger 'Belldandy's' breasts are in page 24, and her marking changes back to what it normally looks like (a triangle).