The Queue based instructions are MEMORY based, not VALUE based. Hence, the string user block you are using for your queue must also change in size from the max length of SL to SS (256 max chars to 64 max chars) or vice versa.
You should ALWAYS get that error, unless you simultaneously change the Queue User Block parameter as part of the same multi-variable replace operation, e.g. from MyLongString with MyShortString when also replacing the Value parameter SL to SS.