Yes you can upgraded to enterprise to do that
http://hellomate.typepad.com/exchange/2003/07/upgrading_stand.html or
http://www.petri.co.il/upgrading_from_exchange_server_standard_edition_to_the_enterprise_edition.htm. Second part yes you can do that once you have two stores, to do this
http://www.petri.co.il/upgrading_from_exchange_server_standard_edition_to_the_enterprise_edition.htmMake sure the user are not logged into outlook or outlook web when you do move them becaus if you do it will currupt their box and thats not fun to fix.