This should work with your sample data.
select *
from CustomerConditions
where IsActive = 1
order by SortOrder
begin transaction
update CustomerConditions
set SortOrder = x.RowNum
from CustomerConditions c
cross apply
(
select ConditionID, ROW_NUMBER() over (order by isnull(SortOrder, 9999999), ConditionID) - 1 as RowNum
from CustomerConditions
where IsActive = 1
) x
where x.ConditionID = c.ConditionID
select *
from CustomerConditions
where IsActive = 1
order by SortOrder
rollback transaction
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