Assuming you don't need the old vCenter Server's data (history, events, resource pools settings, permissions, ...) and further assuming the hosts aren't compatible anyway, I'd go ahead and install the new hosts as well as the new vCenter Server and present the datastores to the new hosts. Then shutdown the VM's on the IBM hosts, detach them from the inventory, attach them to the new hosts' inventory and power them on. Make sure you select "I moved it" when asked whether moved or copied. Once done you may want to upgrade VMware Tools and the VM's hardware version (compatibility mode).
André