substring in stored procedure problem

We have ONLINE 5.0 and are trying to create an upshift function in a stored procedure. However, when we try something like: for i in (1 to leng STEP 1) IF value[i,1] = 'a' then let value [i,1] = 'A'; ELIF value[i,1] = 'b' then let value [i,1] = 'B'; . . ELIF value[i,1] = 'z' then let value [i,1] = 'Z'; END IF end for; We get a syntax error because substrings value[..] doesn't allow variables in the index. Is there any way around this problem.