Virtual hardware upgrade to 09 (ESXi 5.1) from 4 (ESX 3.5)?

Hi guys

Somebody made an upgrade form Virtual hardware from VMs with 4 version, to 9??

Is there issues? i have to upgrade 9 VM win2003 , 1 vm win2000 and 1 vm with windows Net, somebody knows if there are issues?Wich is the process to do it?

Thanks & Regards