OWA Redirection and DNS Settings during Exchange migration to 2013

Hi,

I am planning to migrate user from Exhange 2010 to exchange 2013 server which i setup recently. I have below queries before i move the users.

1. how can they will access OWA after moving to Exchange 2013 with co-existence environment.

2. which OWA site will be primary at this stage (Exchange 2010 or 2013)? 

3. how should i set proxy for OWA redirection when user trying to access other version site.

4. What all DNS changes internally and external i need to perform to move all the services to Exchange 2013 server. 

any help would be appreciated.

Thanks, Karthikeyan R