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

I know the problem is how MS Project does the time conversion.  My master schedule consists of importing schedules from 6 external contractors.  Four of them use 7 hr/day, 35 hr/wk calendars and two of them using 8hr/day, 40 hr/wk calendars.  My organization also uses the 7h/d-35h/w calendar.  

What I am trying to accomplish is to still have the same finish date on the tasks when I import the outside schedules which use the 8h/d, 40h/w calendar.  

I want 1 day to equal 1 day in the master schedule.  We do not break tasks down to hours.  It does not matter to me if one of the contractors work 8hr/day versus 7 hr/day.  Our organization has not yet begun to measure work effort on a per hour basis.  

If the complete work package was 80 days in their schedule, I want it to also say 80 days when imported into mine – not 91.43 days.  The most important thing to our project right now is have a correct completion date.
 

OK. I hoped this would be the case. You are basically interested in Start Date and Finish Date. Only the funny Duration you should like to be changed? I hope nobody is using extra Task Calendars? How do you use the external planning data? Copy/paste or really import as Sub-projects? You don't have to return schedules, only you want a complete overview? Depending on the way in which you use the external data there may be some tricks to add a Custom Field showing the Duration in your Working days. It would require a Flag field and a Text Field. Is it OK if everything would be displayed in days and not a mixture of days, weeks and so on? Still you would have to add the two fields to the projects you want to import as Sub-projects. But then you get the Duration again in "normal" days. Are you, or your subcontractors using Custom Fields? Which ones in Text and Flag. If there are still free I could give you an example hoe to go about it.
In e.g. Text20 I would use the following formula:
IIf([Flag20];ProjDurConv(ProjDurValue([Duration]*7/8);7);ProjDurConv(ProjDurValue([Duration]);7))
It may be necessary to replace ; by , depending on your list separator.
Make certain that the same formula would be used for Summary Tasks. [Tick box]
In all projects the fields Text20 and Flag20 have to be added. If you have an 8 hours calendar set Flag20 to Yes. This is assuming you use a 7 hour one.
Rename Text20 e.g. to Dur. Check everything and then in your table remove [hide] the fields Flag20 and Duration or move these far to the right of the table.

Why an easy solution if you can make it complicated?
Greetings from the Netherlands

You may have to experiment with the correct value for Flag20 in your master project. It is rather easy to give a field the same value in several consecutive rows.

Why an easy solution if you can make it complicated?
Greetings from the Netherlands

I'm very new to MS Project so, I'm not sure what an extra task calendar is so, I don't know if one is used.  The external planning data is just get get full picture of project (including all the sub-contractors).  The external data is import as sub-projects (well, I'm using MS Project 2007 so, it's inserted as schedule).

I do not know what flag field and text field are or how to use them so, I will have to do some research on your suggestion.  Thanks so much for your assistance so far.
 

Have you in mean time discovered what Custom Fields are and how to use these? Or should I try to upload a few example files? However, this may take some time as I am very busy next week.

Why an easy solution if you can make it complicated?
Greetings from the Netherlands

I have had some other pressing deadlines so, I haven't been able to play around with these custom fields.  I'm sure I will be able to figure it out when I dedicate some time to it.  Thank you for your help.  I will write a message if I have an issue though.