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Thanks for your comment<br />
Research confirms that most problems would be related to partial migrations, should be able to complete this task in stages
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I am planning to install Exchange Server 2013 on a Server running 2012R2; there is an existing instance of Exchange 2010 running on another Server. This is my first Exchange 2013 migration. If there are problems with installation, setup or configuration, will there be a conflict running 2013 until these are resolved? What is the Window for migration? How long can I run 2013 before updating firewall, and begin final migration?
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I am planning to install Exchange Server 2013 on a Server running 2012R2; there is an existing instance of Exchange 2010 running on another Server. This is my first Exchange 2013 migration. If there are problems with installation, setup or configuration, will there be a conflict running 2013 until these are resolved? What is the Window for migration? How long can I run 2013 before updating firewall, and begin final migration?

Run Exchange 2013 with existing Exchange 2010 Server

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

IT Administration is the processes and best practices for programming and development, and incorporates methodologies for managing activities and projects. Common methodologies include waterfall, prototyping, iterative and incremental development, spiral development, rapid application development, extreme programming and various types of agile methodology. The life-cycle "model" is a more general term for a category of methodologies, and a software development "process" a more specific term to refer to a specific process chosen by a specific organization.

IT Administration

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. A MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol (SMTP). The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.

Email Servers

Windows Server 2012 is the server version of Windows 8 and the successor to Windows Server 2008 R2. Windows Server 2012 is the first version of Windows Server to have no support for Itanium-based computers since Windows NT 4.0. Windows Server 2012, now in its second release (Windows Server 2012 Release 2) includes Foundation, Essentials, Standard and Datacenter, and does not support IA-32 or IA-64 processors.