can you replace the transmission in a 2006 BMW 750LI or would it be best to rebuild it

The 2006 BMW transmission is electronic and can have many causes for not operating correctly. Much more information is needed to answer your question. I would advise a qualified technician looks at it and performs diagnostics. A specialty scanner would be needed to communicate with the transmission control module. If the symptom is exactly the same after the transmission replacement , then the diagnosis may have been incorrect or the repair may have not been performed correctly.