We can make transition in FOLLOWING options :

SINGLE FOREST to single forest you have an existing single forest Exchange 2003 or Exchange 2000 topology, you can transition to a single forest Exchange 2007 organization Single forest to cross forest sIf you have an existing single forest Exchange 2003 or Exchange 2000 topology, you can transition to a cross-forest Exchange 2007 topology

Cross forest to cross forest If you have an existing cross-forest Exchange 2003 or Exchange 2000 topology with Exchange SERVERS and mailboxes in each forest, you can transition to an Exchange 2007 cross-forest topology.

Resource forest to resource forest –
Single forest to resource forest –

We can make transition in following options :

Single forest to single forest you have an existing single forest Exchange 2003 or Exchange 2000 topology, you can transition to a single forest Exchange 2007 organization Single forest to cross forest sIf you have an existing single forest Exchange 2003 or Exchange 2000 topology, you can transition to a cross-forest Exchange 2007 topology

Cross forest to cross forest If you have an existing cross-forest Exchange 2003 or Exchange 2000 topology with Exchange servers and mailboxes in each forest, you can transition to an Exchange 2007 cross-forest topology.

Resource forest to resource forest –
Single forest to resource forest –