We make a schedule using 8 hours/day calendar.
Now we want to change to 10 hours/day calendar.
We can see the Activity Duration now change to 8 days and Project Completion date is now earlier.
How can we change Hours/day but keep duration unchanged?
I will show you how to do it.
Go to Tool -> Global change. Click New.
Enter Data as below picture:
Click on Change button. Primavera will show us a Preview. Then click on Commit Change
Now we can see Activity Duration change back to 10 days and Project completion date is the same with original one.