The token distribution isn't going to change - the way Cassandra maps replicas will change.
How many data centers/regions will you have when you're done? What's your RF now? You definitely need to run repair before you ALTER, but you've got a bit of a race here between the repairs and the ALTER, which you MAY be able to work around if we know more about your cluster.
How many nodes
How many regions
How many replicas per region when you're done?