An efficient way to maintain rotation stand-by shifts

I have the following:

- 1 calendar per country
- 1 general shift definition per country
- 50 engineers per country (example)

Now I have these 50 engineers working Mo - Fr, from 08:00 - 17:00 hours.
Every weekend there is a stand-by shift, that all engineers have to work every 6 weeks or so.

Question: how do I maintain those stand-by shift in an easy way. I don't want to manually create trips for engineers in the dispatch center.
I do want to be able to quickly change two engineer's stand-by shifts, because they "traded".

So what's the easiest way of doing this?

ideas/tips?