Hi Bustos,
I would like to answer the questions one by one:
Q1: Is there any impact to the existing Office 365 login/environment by enabling Windows Azure AD for all my existing services in preparation?
A1: It depends on whether you have started to use Office 365 services. For example, if you have built many mailboxes that have the same SMTP address as the local mailboxes on Office 365, after enabling Directory Synchronization (DirSync), it will cause impacts.
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Q2: Is there any impact to existing on-premise login of domain.local\userid after adding the domain suffix? By that I mean can users still log in with domain\userid? Or will they need to log in with the userid@publicdomain.com?
A2: The short answer is no. After adding another UPN suffix:
1. On the local AD side, end users can still use “domain.local\userid” to access the local AD services.
2. On the Office 365 side without Single Sign-On (SSO), end users can only use their Office 365 UPN, such as “userid@publicdomain.com”, to access Office 365 services.
3. On the Office 365 side with SSO, end users can use “domain.local\userid” to access Office 365 services.
As a tip, since you want to enable DirSync, no matter SSO is used or not, the local AD users’ UPN suffix needs to be changed to a public domain, such as “userid@publicdomain.com”.
Thanks,
Claud