>Because Delete does not delete the records in my detail table which I use foriegn key to cascade delete my detail table.
Ok.
You want to change the primary key to 1 of the unique string by calling SYS(2015)?
Ok, but as I can see, you first replace primary key and then delete it... delete command cannot find it because it has new value (SYS(2015))

Try :
TmpUserCode = SYS(2015)
replace usercode with TmpUserCode
delete from tsmtranh where usercode = TmpUserCode