Hi JustMe666,
Basically, the DNS record settings can be updated automatically during the Office 365 service upgrade. However, we do receive information from customers that some Office 365 tenants/domains have issues after service upgrade, and that is why we suggest customer track down whether your Autodiscover DNS settings are set up to work with Office 365 email after you’re upgraded.
If the Outlook client keeps asking the credential, I'd like to suggest you refer to the following steps:
1. Re-run the desktop setup for Office 365.
2. Check the Outlook client setting:
a. Open Outlook client. Click File->Account Settings->Account Settings.
b. Click the E-mail tab, click the Exchange Online mailbox, click Change…
c. Click More Settings…. Click the Security tab, make sure the Always prompt for logon credentials box is unchecked.
Thanks,
Bourne Zhang