Set replica set member host names on Secondary
There is a guide for how to do this (reconfigure a replica set with unavailable members) here:

http://docs.mongodb.org/manual/tutorial/reconfigure-replica-set-with-unavailable-members/

You essentially have to do a forced reconfig of the set because there is no primary available.  If you want to keep a set operational while doing this type of move, you can: 


Use your hosts file to temporarily point the old names to the new (or similar) and "fool" the primary into thinking they are up (then make the permanent changes one at a time)
Temporarily add arbiters to prevent the primary from stepping down (2 more for example, so the primary is kept with 3/5 votes)
Reconfigure the set to a single node before doing the moves
Do the moves one server at a time, ensuring there are 2/3 nodes up to form a majority at all times (and hence keep a primary up)