save_post Runs whenever a post or page is created or updated, which
could be from an import, post/page edit form, xmlrpc, or post by
email. Action function arguments: post ID and post object. Runs after
the data is saved to the database.
above paragraph is quoted from WP Codex.
so you cannot use this hook to get older value because it fires after saving new values to DB. WP has another action hook named
wp_insert_postbut sadly this hook does same thing as
alternatively you can use Filters to get the job done. WP provides few filter to edit the post while saving or before saving to DB. like
content_save_pre might work for you, i think.
here is another discussionon this topic which might be helpful for you.