Please........There is no question here......
You DO NOT take the game!!!..........you do not do the bases or the plate.....you can not ever change the fact you used to be the head coach of that team. That point alone will be known by enough people to bring every call under suspicion...........even if you nail every call, each call that goes for your old team will be suspect.
The only thing we as umpires have is our personal integrity....you may be able to umpire that game without a hint of bias, but why would you subject yourself to that???
Call the assignor, they should not have given you this game......, they should be happy to transfer you to another game....if not , beg out of the assignment, if you are good enough for this tournamnet assignment, you will get another one....
For my money there is no good that can come out of you doing this game........
as for the selection process..........well, you have to take control of it in this issue.....the assignor will be happy you called this to his attention, the last thing he wants is complaints for alleged umpire bias.......