SELECT REPLACE(REPLACE(REPLACE(@STRING, 'BA#', '***'), 'A#', 'C#'), '***', 'BA#')

with MyTable(ID,SomeString)
AS
(
SELECT 1,'A#+AB#+BA#+B#' UNION ALL
SELECT 2,'C#+AB#+BC#+B#'
)
select * from MyTable
cross apply dbo.delimitedsplit8k(SomeString,'#')

ID   SomeString       ItemNumber    Item
1    A#+AB#+BA#+B#    1             A
1    A#+AB#+BA#+B#    2             +AB
1    A#+AB#+BA#+B#    3             +BA
1    A#+AB#+BA#+B#    4             +B
1    A#+AB#+BA#+B#    5             
2    C#+AB#+BC#+B#    1             C
2    C#+AB#+BC#+B#    2             +AB
2    C#+AB#+BC#+B#    3             +BC
2    C#+AB#+BC#+B#    4             +B
2    C#+AB#+BC#+B#    5       

SELECT STUFF( REPLACE( '+' + string, '+A#', '+C#'), 1, 1, '')
FROM(SELECT 'A#+AB#+BA#+B#' AS string) A

with MyTable(ID,SomeString)
AS
(
SELECT 1,'L@*K@*H@/324' UNION ALL
SELECT 2,'(AF#-AR#)*(Y#+BB#)' UNION ALL
SELECT 3,'TT#*(L#+D@)*C@' UNION ALL
SELECT 4,'((AE#-AR#)*(Y#+A#))/Y#/'
),
MyFn (fnval) AS
(
SELECT '@' UNION ALL
SELECT '&' UNION ALL
SELECT '#'
),
MyReplaceMents (oldval,newval)
AS
(
SELECT 'L','AT'
)

select 
  MyTable.* ,
  REPLACE(SomeString,oldval + MyFn.fnval ,newval + MyFn.fnval ) AS NEWVAL
  FROM MyTable
  CROSS JOIN MyFn
  CROSS JOIN MyReplaceMents

ID SomeString               NEWVAL
--------------------------------------------------
1  L@*K@*H@/324             AT@*K@*H@/324
2  (AF#-AR#)*(Y#+BB#)	    (AF#-AR#)*(Y#+BB#)
3  TT#*(L#+D@)*C@           TT#*(L#+D@)*C@
4  ((AE#-AR#)*(Y#+A#))/Y#   ((AE#-AR#)*(Y#+A#))/Y#
1  L@*K@*H@/324             L@*K@*H@/324
2  (AF#-AR#)*(Y#+BB#)       (AF#-AR#)*(Y#+BB#)
3  TT#*(L#+D@)*C@           TT#*(L#+D@)*C@
4  ((AE#-AR#)*(Y#+A#))/Y#   ((AE#-AR#)*(Y#+A#))/Y#
1  L@*K@*H@/324             L@*K@*H@/324
2  (AF#-AR#)*(Y#+BB#)       (AF#-AR#)*(Y#+BB#)
3  TT#*(L#+D@)*C@           TT#*(AT#+D@)*C@
4  ((AE#-AR#)*(Y#+A#))/Y#   ((AE#-AR#)*(Y#+A#))/Y#

,cte_Replace AS
(select 
  MyTable.* ,
  REPLACE(SomeString,oldval + MyFn.fnval ,newval + MyFn.fnval ) AS NEWVAL
  FROM MyTable
  CROSS JOIN MyFn
  CROSS JOIN MyReplaceMents)
 SELECT * FROM cte_Replace
 WHERE SomeString != NEWVAL

MyReplaceMents (oldval,newval)
AS
(
SELECT 'L','AT'
UNION
SELECT 'AR','ZZ'
),

DECLARE	@Searched	char(1),
		@New		char(1)
SET @Searched = 'L'
SET @New = 'M'
DECLARE @Test	TABLE(
	string		varchar(1000))

INSERT @Test
SELECT 'L@*K@*H@/324' AS string
UNION ALL SELECT '(AF#-AR#)*(Y#+BB#)'
UNION ALL SELECT 'TT#*(L#+D@)*L@'
UNION ALL SELECT '((AE#-AR#)*(Y#+A#))/Y#';

WITH cteTally(n) AS ( SELECT TOP 1000 ROW_NUMBER() OVER( Order BY (SELECT NULL)) FROM master.sys.all_columns),
Data AS( 
	SELECT '+' + string string, n, SUBSTRING( '+' + string, n, 1) ch
	FROM @Test, cteTally)
UPDATE t SET string = STUFF( REPLACE( d.string, dp.ch+d.ch+dn.ch, dp.ch+@New+dn.ch),1,1,'')
FROM @Test t
JOIN Data d ON '+' + t.string = d.string
JOIN Data dp ON d.string = dp.string 
			AND d.n-1 = dp.n 
			AND dp.ch LIKE '[^A-Z]'
JOIN Data dn ON d.string = dn.string 
			AND d.n+1 = dn.n 
			AND dn.ch IN ( '#', '@', '&')
WHERE d.ch = @Searched

SELECT * FROM @Test