It's possible that your YGroup has been "advertised" on many other websites, so just killing it entirely *might* be bad.? I suggest editing the home page/welcome page/advertising page on Yahoo to state at the group has moved and include a link in plain text to the new group.? ?In case yahoo disables html, new people could maybe copy/paste the link, so make it visible like that.
To check how it displays, you can open a different browser or a private/incognito session and go to the group page without logging in.