UPDATE tbl
SET    col = replace(col, 'l', 'L')

Or if there other lowercase as well that should be uppercase:

UPDATE tbl SET col = upper(col)

I must be doing something wrong, this is my error.

Msg 547, Level 16, State 0, Line 3
The UPDATE statement conflicted with the REFERENCE constraint
"FK_GL_TrnsLne_IMA". The conflict occurred in database "EAGLE", table
"dbo.GL_TrnsLne", column 'GLL_ItmID'.
The statement has been terminated.

You would use a constraint which forces upper case in the column.
Don't forget to take into account of the column collation.
The next T-SQL script makes the point more clear.

CREATE TABLE #foo_cs(
  col CHAR(7) COLLATE Latin1_General_CS_AI);
GO
CREATE TABLE #foo_ci(
  col CHAR(7) COLLATE Latin1_General_CI_AI);
GO
ALTER TABLE #foo_cs
 ADD CONSTRAINT uppercase_always_cs
     CHECK(col = UPPER(col));
GO
ALTER TABLE #foo_ci
 ADD CONSTRAINT uppercase_always_ci
     CHECK(col COLLATE Latin1_General_CS_AI = UPPER(col));
GO
INSERT INTO #foo_cs(col) VALUES('99l9999');
GO
INSERT INTO #foo_ci(col) VALUES('99l9999');
GO
INSERT INTO #foo_cs(col) VALUES('99L9999');
GO
INSERT INTO #foo_ci(col) VALUES('99L9999');
GO
SELECT col FROM #foo_cs
WHERE col COLLATE Latin1_General_CI_AI = '99l9999';
GO
SELECT col FROM #foo_ci WHERE col = '99l9999';
GO

DROP TAble #foo_cs, #foo_ci;
GO

--
Andrey Odegov
avodeGOV@yandex.ru
(remove GOV to respond)

Sorry, Let me start over.  I am trying to do a mass change of a bunch of
numbers that a user entered incorrectly.  He entered them with a lower case
letter instead of uppercase.  The numbers all begin with a 99l and I want to
do a mass replace to change them all to a 99L.  The numbers have a suffix
that is not constant like the prefix.  Example; 99l555, 99l556, 99l557 and
so on.

The name of the table is dbo.item and the column is ima_itemid.  Here is my
select statement.

Select * from dbo.item where
 ima_itemid like '99l%'

This result gives me a mixture of 99l's and 99L's.  So apparently the user
eventually started entering them the correct way, in case that is an issue
having a mixture.  It appears that my select statement doesn't care about
case since I get upper and lower, although I used lower in my select.  Hope
this clarifies some.