I'm trying to find the easiest way to replace a resource on a task that is partially completed.

e.g if Bill and David are 75% through their task and David goes off sick I only have two other resources available. Simon and Tony.
They are also 75% through their task but I need to pull Simon and place him with Bill for the duration of their task. Tony will have to complete the task on his own as I have no other resources available.

Any idea which is the easiest way to do this as I have to teach someone how to do it and I'm struggling myself!

Many thanks, Lol