Hi Lars,
(Did we meet on Holland?)
The value stored in the database does reflect the decimal value. If you have a quantity of 1 and display decimals of 4, then the value stored on the database is 10000. So changing to 6 decimals, the value would then translate as 0.01 unless you update everywhere that the field is used.
This process is best performed at the install stage, before any data is in the files.
If you are doing this further down the line, then you should build the search indices, so that you can search for the tables that the DD element is used in, and update them by whatever means is convenient. An SQL script is probably easiest and quickest, as all of this would have to be done offline.
Testing these sort of changes is difficult as most people are running only one Data Dictionary, so the change will affect all environments at the same time.
Cheers,
David