Replication between two different SQL Server versions
It sort of depends whether you're using Database Mirroring, or Transaction Replication (although I suppose you could be using both)

Assuming Transaction Replication:

This is supported, so long as you stick to certain rules, as described in the MSDN article Upgrade Replicated Databases 

As long as your Distributor is the same version or greater than your Publisher, and a Subscriber is within two versions of the Publisher, which fits in your case so you're OK. That MSDN article provides instructions. 

This is similar to the question/answer asked a couple of years back, but the MSDN article has been updated since. 

If you mean Database Mirroring:

I'm assuming you don't mean this, as this question has been asked before in a number of ways, but I'll add the answer in for completeness. 

This can be done, but it is supported strictly for minimising downtime during upgrades (which it seems is what you are doing, so that's OK). 

Have a look at MSDN - Minimize Downtime for Mirrored Databases When Upgrading Server Instances for information about this (and a walk-through).