unless you are looking at the TIMES meaning what time an activity starts and what TIME an activity finishes it really does not matter becasue a 1 day activty starts on that day and finishes on that day it does not matter if you work 8 hours per day or 10 hours per day it is all still that same day. 

Now the difference comes into play if you are using manhours to determine hoow many men that you need per day.

if you are using the duration type of fixed duration/units time then all lyou have to do is to make sure that you have a calendar that is 8 hours then filter for all activiites then add a column for the calendar (under general) then assign the firat activity to that clanedar in the column then do a fill down for teh rmainder of teh activities. This assigns your activities to the proper calendar.

The other thing that you need to do is to go to the project directory and and make sure that you project is highigthed click on the defaults tab and on the right had side choose the 8 hours per day calendar. This way any new activities added to the scheudle wil automatically be assigend to the correct calendar.

if you are using an duration type that that determines your duration based on manhours then you have anotehr problem. 

Do you have resources as well? If not, then you can do a global change.  Please make sure you backup a copy of the programme and baselines before doing this.  You will also need to manually extract the baseline.  If you do have resources etc, you will need to extract the programme to excel and manually change it there - I've personally not been in that situation as yet so I've not made a work around.

10/8 = 1.25

Therefore we need to divide all activities by 1.25.  But you cannot divide/multiple by fractions in P6 v8.2 (integers only, not floating point numbers).  Therefore we have to increase the original duration by a factor of 100 so that we can divide by 125.

Under the 'Where' section, you need to save the remaining duration after the first calculation.  You need to use the {custom} field to allow you to do maths.

I have used duration 1

Line 1 of "Then" section of global change.

Parameter:  Duration 1

Is: =

Parameter/Value: Remaining duration

Operator: *

Parameter/Value: {custom} (see below for value)

custom in this case is 100

2nd line

Then

Parameter: Remaining duration

Is: =

Parameter/Value: remaining duration

Operator: /

Parameter/Value: {Custom} set to 125

Apply the global change

You can now change the calendar to 8hrs and reschedule.

Anyone got a better way of doing it, give me a shout I always like to learn new things...

I've not played with changing units etc, but the below seems to work - I've bound to have forgotten something though...

You could change subject area in the global change to 'activity resource assignments' (top left box when you've created a new global change) - don't forget to backup just in case and make sure you check it's applied properly before doing lots of major changes.

Where (nothing)

Then:

Parameter: Number 1

Is: =

Parameter/value: Remaining units/time

Operator: *

Parameter/Value: 0.80 (numbers are floating point, durations are not)

AND

Parameter: Remaining units/time

Is: =

Parameter/Value: Number 1

Should do the trick assuming you've not progressed anything.