Plan run customized, but after certain task is finished

Need to change some plan variables at build specific execution time. If there's a feature at run customized pop-up window that says: wan to run costumized starting from a specific task(otherwise it runs normal). for my example, I have a plan that runs with specific plan varaibles, and I need some of those variables to be changed after some task is finished, and then I would like next task to use those newly changed variables.