How can we change an activity duration before the activity is started?

Summary:

We want to change the activity duration [length] before the technician has started the activity. This could also be done before the activity is routed to a resource. Since OFSC continuously recalculates activity duration, it would mean that if the duration is changed "manually" by a user, then OFSC should no longer calculate the duration of the activity.

Content (required):

We want to change the activity duration [length] before the technician has started the activity. This could also be done before the activity is routed to a resource. Since OFSC continuously recalculates activity duration, it would mean that if the duration is changed "manually" by a user, then OFSC should no longer calculate the duration of the activity.