DB2 - Using stored procedure variable in a "CREATE TABLE as (..)" Statement
The use of an intermediary variable is not necessary here. Db2 regsitry variables, including CURRENT TIMESTAMP or its standard-compliant variant CURRENT_TIMESTAMP, are set once within the scope of a statement referencing them. This means that in a statement like yours:
SELECT   
    x,  
    y,  
    CURRENT_TIMESTAMP as t_change  
FROM  
    bar 

all rows will have the same value of t_change as of approximately the beginning of the statement execution.