Use of SBS 2008 after migration to SBS 2011

I have been unable to find a straightforward answer to the following situation. I'm migrating from SBS 2008 to SBS 2011, after the migration we want to keep using the old SBS 2008 to host our website. I plan to put the SBS 2008 on to it's own network after the migration but I was not planning running the DCPROMO before removing it or even removing Exchange 2007 just going to delete the mailbox's. I thought that this would work just fine but I'm starting to read that the old SBS 2008 server will start shutting itself down after 21 days. Is this still going to be the case if it is once again on it's own network? If so is there any way to correct it without doing a complete reinstall of SBS 2008? I guess worse case I can just do a reinstall using the backup prior to the start of the migration?