Check out ZeXtras Migration Tool, a free tool for Zimbra to Zimbra migrations..
With it the steps for your migration would be fairly straightforward:
- Upgrade your Debian server to Zimbra 6.0.7+ (required by the tool)
- Prepare your Ubuntu server, install Zimbra 7.1.1 on it and configure it (same configuration than the Debian server)
- Create your domains on the new server and use Zextras Migration Tool to move your data to your new server.
You can find the ZeXtras Migration Tool in the Zimbra Gallery:
ZeXtras Migration Tool | Zimbra :: Gallery
Have a nice day,
Cine - Community Manager - ZeXtras
www.zextras.com