I want to update the master column in my detail IG but I can't. When the page submits the old value returns. All the other columns updates as expected.
Any suggestions?
Basically, you want to move detail row to another parent, right?
It seems that you can't do that in a master-detail form because that column is used to enforce referential integrity; in detail block, its "Master column" property is set to master region's primary key column.
Initially, detail primary key item is hidden. It means that it is always inherited from the selected master record. If you add a new row, it'll belong to that master. So, if you unhide it (and set it to e.g. text item) so that you'd be able to update its value, processing "reverts" your changes and sets the detail region primary key value to the same selected master's primary key value.
What to do? Create another page which will be based on detail table, then there won't be any restrictions of that kind (i.e. you'll be able to change the primary key column to point to another master).