Duration Changes to Decimal figures

If you want Durations to stay "in place" as best as they possibly can, then you should leverage "Fixed Duration" and "Not Effort Driven".

1. Change your default options so that any new tasks are set this way:
Click on >Tools, >Options, >Schedule Tab
Select "Fixed Duration" for the Default task type.
Uncheck the option New tasks are effort driven

2. Change these options for existing tasks
Insert the column Type. Change all to Fixed Duration.
Insert the column Effort Driven. Change all to No.

Now Durations should hold when you add resources or establish constraints. (use caution in these constraints. You are eliminating the dynamic scheduling capabilities of Microsoft Project).

Hope this helps!
J Black
http://www.pmconnection.com