Change data type for select-option while runtime

Hey Guys,

I have a quit difficult problem. I created a report with variable select options and using the SAP standart process to select fields from a customer table for selection (like in SE16). This works fine for me. But the problem is, that I would like to create the report as usefull as possible and so I want to allow user to change the selection fields while the selection screen is displayed. For example:

SELECT-OPTION: SO1 LIKE SEL1.

INITIALISATION.

SEL1 = '/SAPAPO/MATKEY-MATID'.

This works fine! But know I want to change the data type after it was generated:

SELECT-OPTION: SO1 LIKE SEL1.

INITIALISATION.

SEL1 = '/SAPAPO/MATKEY-MATID'.

SELECTION SCREEN OUTPUT.

SEL1 = '/SAPAPO/MATKEY-MATNR'.

Here is the problem. I dont get any error message or something like that, but nothing happends. I guess it is because I already signed a data type to the select option SO1 and it does not overwrite this. Is there any chance to modify the select options while the selection screen is displayed?

Thanks

Andy