Correct. Duration is always measured in working time. So, for example:

The task was originally 4 days long - starting on Wednesday and finishing on the following Monday. Four working days counting Wednesday, Thursday, Friday, and Monday. Saturday and Sunday are not counted in the "4 days" because, unless you have changed the calendar, Saturday and Sunday are non-working days.

You then change the calendar and let Saturday be a working day. The task duration is still 4 days - but now ends on Saturday because Saturday is now a working day.