Serial attribute and org attribute problems with item master

Question

IF i want to unassign an item from an org, how would i do tht
The check is not updateable in the organization assignment tab
Secondly:
When i try to make an existing item serial controlled, i go to inventory tab on item master and when i change the serial control attribute GENERATION then "For some items it says APP-INV-05525: Either On Hand quantity or pending transactions exist for item, cannot change this value"
But for same item if i choose GENERATION sales order issue than it works fine.
Thirdly:
How would i Use generated serial numbers..how to assign them or use them for issuing and recieving

Answer 1

Regarding the 1st part of your query: Unassigning item from an Org
You cannot unassign an item that way (by unchecking the Assigned checkbox).
You have 2 options:
1) Delete the item from the respective Org if it is not yet transacted in that org.
Navigation : Items-> Delete Items
Once selecetd this will open deletion Group screen, please enter a new group name and description and for type select Item from LOV and then in details Tab enter your item that you want to be taken out.
Save the transaction and then click on Delete group button at the bottom of the screen --> this will raise a concurrent request.
If it is already transacted in the respective org from where you want to delete it then system will not allow to delete the item for audit trail purpose.
2) You can make the item 'Inactive' to prevent transactions or ordering (purchasing/sales) you can uncheck the appropriate attributes on that item in the org (assuming your attributes controls allow it at org level rather than master level).
Regarding Serial number generation options a) At Receipt and b) At Sales Order Issue:
a) At Receipt >> Create and assign serial numbers when you receive the item. As you move the material within inventory and until you issue it out you have to use the serial number for those transactions.
b) At Sales Order Issue >> Create and assign serial numbers when you issue (ship) the item against a sales order. For this option serial numbers are required at ship confirm.
Now as per the definitions given above, if there on-hand already exists in your organization(s) then you cannot set the serial generation control "At Receipt". Because in that case all existing on-hands should have serial number assigned which is not the case. On-hand should be zero to set the control "At Receipt". Hence system will prevent you from setting it to "At Receipt" when on-hamd exists. Also in your post you mentioned that you tried to change the control in Item Master, that means serial control generation attribute is set at Master level for you. In this case it will consider on-hands for that item in all the child orgs.
On the other side if you are setting serial control generation "At Sales Order Issue" then seral number is not required for existing on-hands because it will be created and assigned during ship confirm. So system will not prevent even if on-hand exists.
Considering the options available in the system your third question (How would i Use generated serial numbers..how to assign them or use them for issuing and recieving) not very clear. The "generated serial numbers" part is creating little confusion in mind as to what you are actually looking for.
As explained above, if control is set "At Receipt" >> Create and assign serial numbers when you receive the item. Use the Lot/ Serial button on the right hand corner of the material transaction screen and enter the serial number.
If control is At Sales Order Issue >> Create and assign serial numbers when you issue (ship) the item against a sales order. In the Shipping Transaction screen use Tools >> Serial Numbers option before ship confirm and enter the serial number.
But if you have set the control as "Predefined" then, assign predefined serial numbers (by generating before hand) when you receive the item. Thereafter, for any material transaction, you must provide a serial number for each unit.
Hope this helps.