How to resolve unmodifiable replica set state
When you add hosts, _id is set always one bigger than what is current max _id. If you really want to "fix" that, you can always do:

var cfg=rs.conf()
cfg.member[3]._id=0
cfg.member[4]._id=1
rs.reconfig(cfg)


Same way, you can change anything or everything in your rs configuration. Remove or add new hosts, change an order of hosts in the list.