The major problem is likely that internally MS Project store all durations in minutes and use the calculation factors for hours per day, days per week and days per month to convert the internal stored data in minutes into days if you choose day as the unit for showing duration. So, not the amount in hours will change, only that in days.
The question is what you want to achieve. If resources have been planned you can not just change 40 hours per week into 35 hours per week. It may be, however that you are not interested in details on an hourly basis and just want to think in arbitrary units of so many working days, no matter how many hours a day one is working.
Please, clarify first what you want to achieve before we try to find some workaround. Do you use resources [Work] in hours? What to do with that? Just reduce all work with 12,5% ? Or...?
You could also think the other way round. Make the 40 hours / week your Project calendar and use a special calendar of 35 hours / week for the perhaps few tasks in your master file. However, if you would give in that case a task 10 days with a 7 hours calendar in reality you would get 80 / 7 working days as duration. Why on earth do you use a 7 hours calendar if you want to neglect the 12,5 % difference for other tasks? As a rule of thumb never try to calculate in more detail than about say half the time you want to update your project. Quite often managers want to plan in hours or maximum a few days while updating the project is done once every 2 weeks or month if ever done. The planning has become a paper tiger in that case.
Why an easy solution if you can make it complicated?
Greetings from the Netherlands