Hello John,
Thumb rule, Exchange 2007 supports only universal group to be mail-enabled but Exchange 2003 does so there wouldn’t be any impact on legacy Exchange version (2003) since it supports both type.
Here is a nice discussion and repo about it, for your reference.
Only Universal Groups can be Mail-Enabled in Exchange 2007, Or Not?
http://www.pro-exchange.be/modules.php?name=News&file=print&sid=299
Now to convert Global/Local group to Universal Group, you can use Quest PowerShell tools or once you introduce Exchange 2007 in your environment you can use below command to convert all Global group to Universal (I would suggest you to simulate this in test lab first).
Get-DistributionGroup | where { $_.Grouptype -Like "Global*" } | Set-Group –Universal
Reference:
Need to convert Global groups to Universal groups?
http://blogs.technet.com/sfodel/archive/2008/08/04/need-to-convert-global-groups-to-universal-groups.aspx