Not able to change number of hours per day in Global Paramaeters

You cannot change hours par day if :
- you defined real work format as hours
- some real work exists.
This is because ProjeQtOr always stores day.
So display of hours is always a converted value.
If some resource entered 12 hours for à 12 hours per day value, 1 days is stored.
So if you change hours per day to 10, when resource gets back, he sees 10 hours although he entered 12 hours.

The only "realistic" way change hours per day is to remove all existing real work...

Now there is a workaround :
- change unit for real work to days, save
- refresh parameters screen : now you can change hours per days...
- when it's done, change back unit for real work
But you must be aware that you'll face previously pointed out issue (resource that entered 12 will now see 10)

For example, I have my full time staff member who works 12 hours per day, but my part time staff member who works only 4 hours per day.

This is easy : your part time resources have capacity of 0.33 (well 0.333333333... in fact).
Another way to deal with resources working different times, is to define work days as 10 hours per day.
Then, resoures working 12 hours per day have capacity of 1.2, resources working 4 hours per day have capacity of 0.4

Hours for each resource will be defined as Capacity on the Resource screen.
Capacity is 1 for 1 full day per day.
If you defined 12 hours per day :
- capacity 1 works 12 hours per day
- capacity 0.5 works 6 hours per day
- capacity 0.25 works 3 hours per day
and so on...