I also wanted to change the startup buffer (or spacemacs-buffer as it is called) and I found https://www.reddit.com/r/spacemacs/comments/55p1td/dont_like_the_new_footer_or_customizing_your/ where basically says that the design of the page is done in spacemacs/core/core-spacemacs-buffer.el (https://github.com/syl20bnr/spacemacs/blob/master/core/core-spacemacs-buffer.el). And I did so, I didn't want to see the release-notes (they are meaningles because now all the development is done in the development branch) and I wrote
(defun spacemacs-buffer//notes-insert-release-note ())
in my .spacemacs file in section dospacemacs/user-config