URL Rewriting of Hostname

Then it's not going to work because you need to flip the client back to FarmA so it shows up on the browser. This would lead to a flip-flop between FarmA and FarmB.

However, what you could is 2 stage a transition (This conceptual design):

Create a VIP and assign it pool FarmA.

Apply the following iRule

  
 when HTTP_REQUEST {    
   if { [HTTP::header "Host"] eq "FarmA" } {  snat automap }  
 }  
 

Repoint your DNS over to the new VIP and test to make the traffic is making it across. Now you have successfully transition over to the F5.

The next step is when you migrate which is simply updating the iRULE and change the virtual default pool to FarmB

  
 when HTTP_REQUEST {    
   if { [HTTP::header "Host"] eq "FarmA" } {    
     HTTP::header replace "Host" "FarmB"   
   }  
 }  
 

At least in concept it should work

CB