It sounds like you're in the same boat I am at the moment in regards to different subnets.
What version of Exchange are you running? For us, we are just adding a couple DAG members up there and using the built-in Exchange method of DR. This to me takes SRM out of the equation and makes for a simpler DR scenario (much quicker too).
If that isn't possible, my initial reaction to you changing the IP address of Exchange is that it may work, as Exchange has its own internal IP addresses it uses for some things so it may not care too much about what it in reality has. If you have the ability, you could build a quick test environment for this and see how it reacts. I did some quick research and came across this link in a technet forum: http://www.zerohoursleep.com/2010/01/5-things-to-consider-when-changing-the-ip-of-an-exchange-server/
I do not believe there is any way of getting around the IP address change, unless you do some sort of layer 2 trickery. I did some initial research on this and everything I read lead me towards using the Cisco Nexus switches for that, which we don't have unfortunately. I am not as well versed in the advanced networking segments to comment more on this.
I think you're on the right track to accomplishing your DR failover.