The OptiPlex 620 can run either Windows 8.1 32 Bit or Windows 10 32 Bit at a push. The system requirements were elevated with Windows 8.1 64 Bit making the processor in the machine not compatible. Windows 8 has reached end of life so is not worth considering. I would advise getting a second hand OptiPlex 790 which are cheap to come by now.
I have a Dell Optiplex 620 running windows 7.  Can this computer be upgraded to Windows 8 or Windows 8.1  My e-mail address is 
Microsoft windows 8