PsyBlog explains you enjoy an experience more when you know it’s going to end. You're more motivated to enjoy the time remaining. You feel happier afterwards.
Your community can have many ends. Yankee stadium’s last night did brought fans together better than their victories. The fans knew it was coming. They paid higher prices for tickets and got plenty of closure.
If you’re moving site, that’s the end of an era. If a key member is leaving, that’s the end of an era. If you’re about to grow beyond your original membership, that’s the end of an era.
Remind members in advance the end is coming, offer plenty of closure, then begin the new era afresh.