This post is based very much on my experiance upgrading several environments from 2003 to 2010 (via 2007 & going directly)
After having massive issues with out-dated and malfunctioning GAL I came across this post:
http://blogs.technet.com/b/exchange/archive/2007/01/11/3397719.aspx
It basically states a bunch of command that you will need to run in order for things to work properly.
Set-EmailAddressPolicy “Default Policy” -IncludedRecipients AllRecipients -ForceUpgrade
Set-AddressList “All Users” -IncludedRecipients MailboxUsers -ForceUpgrade
Set-AddressList “All Groups” -IncludedRecipients MailGroups -ForceUpgrade
Set-AddressList “All Contacts” -IncludedRecipients MailContacts -ForceUpgrade
Set-AddressList “Public Folders” -RecipientFilter { RecipientType -eq ‘PublicFolder’ } -ForceUpgrade
Set-GlobalAddressList “Default Global Address List” -RecipientFilter {(Alias -ne $null -and (ObjectClass -eq ‘user’ -or ObjectClass -eq ‘contact’ -or ObjectClass -eq ‘msExchSystemMailbox’ -or ObjectClass -eq ‘msExchDynamicDistributionList’ -or ObjectClass -eq ‘group’ -or ObjectClass -eq ‘publicFolder’))} -ForceUpgrade
The above commands work perfectly in a “normal” scenario when you are running with everything on standard. Ofcourse if you added any custom objects you should read into the post linked above to use the approriate get commands to upgrade the rest.