Hi VM,
without AD integration you will just lose some authentication flexibility and management in regards to permission and roles delegation through vCenter/SSO server.
No true vSphere features are bonded or relay on MS AD.
If you have already AD implemented there is no reason not use it...
Yes you can join your current VC to AD once is complete create account or group and assign it admin role on vCenter instance. After that its upon you if you will keep your local account ...best practice and also for security reasons is maintaining user accounts solely in AD.
For the upgrade itself I would recommend you to go for custom install option (simple method work best when you start green-field) avoid simple install method.
For more details refer to this KB at the and of the article there are many useful sources: