I'm not sure I understand the question (though I've not worked extensively in the Project module). Can you share some screenshots that might give us more context to advise you further?
usually in Project Management, when we create Tasks we should have the option to either specify the sequential date and time to be Manual (We specify the start Date and End date for each task) or Automatic (The Start date of the second task automatically inherent the end date of the previous task) so the software will calculate the sequential date and time, also calculate the full days of the whole project.
saying so, when the project manager change the time required of any task, the rest of the tasks to be changed automatically without the need to change each task separately, if you can share your email address I can share with you a snapshot of other project management solution.
I cannot take a private email from an item on these forums, it kind of defeats the purpose of the community being able to see each other's ideas.
It sounds like you need to flag the project record in some way that it can kick off a reprocessing of the schedule for all these tasks. You'd probably also need a flag on the Task level to mark certain tasks as eligible for this process (to exclude the ones already finished as well as the one the PM just extended).
Do you have that automated scheduling built out already for the initial scheduling? If so, what is the complication with simply re-running it?