Use a function to update a column

I want to reformat a string in one column of a table, and put that new value in another column of the same table. I created a UDF, and attempt the UPDATE as so:

UPDATE MYTABLE SET REFORMATTED_STRING = FCN_REFORMAT_STRING(ORIGINAL_STRING);

But I get this error: SAP DBTech JDBC: [7]: feature not supported: cannot support non-constant types

Is this possible to do?

Thanks