You could use a correlated subquery, does the following help?

select *, IsNull((
    select top(1) RetailDate
    from t t2
    where t2.RetailDate > t.RetailDate
    and (t2.brand = t.brand or t2.brand = 'ALL')
    order by t2.RetailDate
), GetDate()) NextChange
from t;

See this fiddle demo

This is gaps and islands problem, one option is to use a difference between two row_numbers to define Brand groups :

MIN() used here to return the next Brand='ALL'

WITH CTE AS (
  SELECT RetailDate, Brand,
       MIN(case when Brand = 'ALL' then RetailDate end) over (order by RetailDate rows between 1 following and unbounded following) as next_success_time,
       LEAD(RetailDate) OVER(PARTITION BY IdBusinessUnit, Brand ORDER BY RetailDate) AS NextChange,
       ROW_NUMBER() OVER(PARTITION BY IdBusinessUnit ORDER BY RetailDate) - ROW_NUMBER() OVER(PARTITION BY IdBusinessUnit, Brand ORDER BY RetailDate) as grp
  FROM  tbl_Params P
),
CTE2 as (
  select *, row_number() over (partition by grp order by RetailDate) as rn
  from CTE
)
SELECT RetailDate, Brand, CASE WHEN next_success_time is null and NextChange is null then GetDate()
               WHEN rn = 2 THEN next_success_time
               ELSE NextChange END AS NextChange
from CTE2
ORDER BY RetailDate

Result :

RetailDate  Brand       NextChange
2023-02-05  B Series    2023-02-07 00:00:00.000
2023-02-07  B Series    2023-02-21 00:00:00.000
2023-02-07  Equity      2023-02-15 00:00:00.000
2023-02-15  Equity      2023-02-21 00:00:00.000
2023-02-21  ALL         2023-04-06 10:34:31.673
2023-03-10  B Series    2023-03-11 00:00:00.000
2023-03-11  B Series    2023-04-06 10:34:31.673

Demo here

Try this one, with interleaves of brands: https://dbfiddle.uk/Sx61yBnt

with data(RetailDate, Brand, IdBusinessUnit) as (
    select convert(DATETIME, '2023-02-05',102), 'B Series', 2 union all
    select convert(DATETIME,'2023-02-07',102), 'B Series', 2 union all
    select convert(DATETIME,'2023-02-07',102), 'Equity', 2 union all
    select convert(DATETIME,'2023-02-08',102), 'B Series', 2 union all
    select convert(DATETIME,'2023-02-15',102), 'Equity', 2 union all
    select convert(DATETIME,'2023-02-21',102), 'ALL', 2 union all
    select convert(DATETIME,'2023-03-10',102), 'B Series', 2 
)
select d.*,
    isnull(least(
        min (case when Brand = 'ALL' then RetailDate end) 
            over(partition by IdBusinessUnit order by RetailDate rows between 1 following and unbounded following),
        isnull(
            lead(RetailDate)  over(partition by IdBusinessUnit, brand order by retaildate),
            getdate()
        )
    ),getdate()) as nextchange
from data d
  order by RetailDate, brand
;