Turn a branch's commits plus a stash into uncommitted work on an older base

To get a new branch whose HEAD is an older ancestor commit but whose working tree holds everything from a later branch tip plus a stash made on that tip: create the branch at the tip, git stash apply stash@{N} there first (it applies cleanly on the commit it was made on), then git reset <old-base> (mixed). Doing reset first and applying the stash after can refuse or conflict because the dirty tree already touches the same files. Files that the dropped commits added show up as untracked, not staged. Verify with git diff --stat <old-tip> excluding those untracked files (it should equal the stash diff) and cmp the untracked files against git show <old-tip>:path. Check which commit each stash was made on (git stash list); a stash made on another branch probably belongs elsewhere.