The number of the sheet may not be changed.

A department number is provided so that several applicants can still submit fiches for the same manager.

On the declarant's sheet you will find the box "Department for XML".

Put a number in there (1-9999, example: 2, 9 or 99), then the tax office will make a distinction with the other submitted sheets.

From the instructions of Belcotax 

You make a distinction by using the department number (zone 1.007). With this free to choose number you distinguish between different (virtual) departments of the same debtor. 

Attention: this same department number must always be used for corrections to the original declarations.

Create the XML again after changing the department number. If the complete transmission is rejected or has not yet been submitted, you can delete the transmission of the created XML.