You may be able to create an Action Rule for this. Not 100% it would work as i don't have a setup like that at the moment with Tasks & child tickets - so you'd want to test it first! - but here's my thought:
Action rule Action Rule Info tab: set it to 'on all updates' and 'only if the criteria did not match before the ticket was updated'
Action rule Criteria tab: Tech is any of [select all of the Techs] AND (second criteria): Status is Open
Action rule Actions tab: Change Status to Assigned
The somewhat obvious caveat here is that since the Criteria for "tech is any of <selected techs>" is explicitly defined in the Action Rule, you'd need to remember to update that Criteria if new Techs are added to the WHD system.