Based on this article: https://technet.microsoft.com/en-us/library/dn931280(v=exchg.150).aspx

I would like to ask if using Scenario 2, with only Dirsync, would I be able to go back and add Exchange if it didn't work the way we needed it?

Do not bring future complexity and administrative efforts. Since you are using Dirsync you have to manage recipients on-premise so you need an Exchange (at least one server). Keep hybrid in case you decide moving back. Since all your mailboxes are migrated and DNS pointed to Office 365 you are safe in case of any on-premise Exchange issues. Look at https://channel9.msdn.com/Events/Ignite/2015/BRK4115

• Edited by Slava Fedenko 16 hours 11 minutes ago

Do not bring future complexity and administrative efforts. Since you are using Dirsync you have to manage recipients on-premise so you need an Exchange (at least one server). Keep hybrid in case you decide moving back. Since all your mailboxes are migrated and DNS pointed to Office 365 you are safe in case of any on-premise Exchange issues. Look at https://channel9.msdn.com/Events/Ignite/2015/BRK4115

• Edited by Slava Fedenko Friday, July 10, 2015 3:16 PM

Do not bring future complexity and administrative efforts. Since you are using Dirsync you have to manage recipients on-premise so you need an Exchange (at least one server). Keep hybrid in case you decide moving back. Since all your mailboxes are migrated and DNS pointed to Office 365 you are safe in case of any on-premise Exchange issues. Look at https://channel9.msdn.com/Events/Ignite/2015/BRK4115

• Edited by Slava Fedenko Friday, July 10, 2015 3:16 PM
• Proposed as answer by Winnie LiangMicrosoft contingent staff, Moderator 4 hours 38 minutes ago

Do not bring future complexity and administrative efforts. Since you are using Dirsync you have to manage recipients on-premise so you need an Exchange (at least one server). Keep hybrid in case you decide moving back. Since all your mailboxes are migrated and DNS pointed to Office 365 you are safe in case of any on-premise Exchange issues. Look at https://channel9.msdn.com/Events/Ignite/2015/BRK4115

• Edited by Slava Fedenko Friday, July 10, 2015 3:16 PM
• Proposed as answer by Winnie LiangMicrosoft contingent staff, Moderator Monday, July 13, 2015 2:50 AM

Hi Jonathan,

Based on my knowledge, if you are sure that your will move the mailbox back to the On-premises Exchange server, we can keep the Exchange server on-premises. Since all configuration and mailboxes have been migrated to Office 365, it is doesn't matter to remine the on-premises Exchange server.

Certainly, if you want to remove the Exchange server for migration then add it back when it is needed, it is ok. Personal suggestion, it is better to remain the Exchange server in case there is any issue after the migration.

Regards,