Also watch out for the datastores.
vSphere 4.x hosts can´t see / use VMFS5 datastores (which you may already using with vSphere 5.0).
I haven´t heard of an way to downgrade VMFS5 to VMFS3. If you have multiple datastores you could move VM´s with Storage vMotion to get one by one free to reformat it with VMFS3.
Regards,
Mario