There are a couple of ways that you can go about this.
The easiest way is to send the "kill" job for every worker that you have. Once they've all been killed, then respawn them. The downside of this method is that you will have to wait until all your workers are dead before you can begin respawning. If you existing script respawns immediately, you'll run into problems here.
Another method is to register a unique task for each of your workers. If, for example, you have two workers, register a task "kill_001" for the first worker, and "kill_002" for the second worker. To kill your workers, determine the unique jobs to start ("kill_001", "kill_002"), and then send them out. Respawned workers should have new unique tasks, i.e. don't register a new job "kill_001" if it hasn't been killed yet. Although this method can require a bit more work, it will allow you to respawn your workers without downtime.