An alternate to @txnelson's solution is shown below. The Get Column Reference message returns a list of references to the columns using the Column() function. It does not accept the same arguments that Get Column Names does. Instead, it accepts a list of column names or a matrix.
By using Get Column Names as the argument, you can supply the desired list of column names to Get Column Reference. Because the resulting list is a list of column references, the Data Type message can be sent to the list.
This example captures a list of column references for all numeric columns in the table and then changes the data type to character.
dt = Open( "$SAMPLE_DATA\Big Class.jmp" );
//returns {Column( "age" ), Column( "height" ), Column( "weight" )}
colRefs = dt << Get Column Reference( dt << Get Column Names( "numeric" ) );
colRefs << Data Type( "Character" );
I hope this is helpful!
Wendy