SQL Server : keep track of a running calculation monthly
Here is my idea:
-I used recursive approach,  - for this , added a Row_number, used this to join previous records 

;WITH cte_Source AS
(SELECT 
    year_month
    ,original_threshold  
    ,manualChange
    ,CONVERT(int,LEFT(year_month,4)) As mYear
    ,CONVERT(int,RIGHT(year_month,2)) As mMonth
    ,ROW_NUMBER()OVER(ORDER BY CONVERT(int,LEFT(year_month,4)),CONVERT(int,RIGHT(year_month,2))) AS RN
FROM
    #Test AS T
)
,cteR AS
(
    SELECT TOP(1)
        T.year_month
        ,T.original_threshold  
        ,T.manualChange
        ,0 AS change
        ,T.original_threshold  +  0 + T.manualChange AS  new_threshold
        ,T.RN
    FROM cte_Source AS T
    --WHERE T.year_month ='2017_01'
    ORDER BY T.mYear * 100 + T.mMonth ASC


    UNION ALL

    SELECT
        S.year_month
        ,S.original_threshold  
        ,S.manualChange
        ,CASE WHEN  S.original_threshold &gt; R.new_threshold THEN R.new_threshold - S.original_threshold ELSE 0 END AS  change
        ,S.original_threshold  
            + CASE WHEN  S.original_threshold &gt; R.new_threshold THEN R.new_threshold - S.original_threshold ELSE 0 END 
            + S.manualChange AS  new_threshold
        ,S.rn AS rn
        FROM cte_Source AS S
            INNER JOIN cteR AS R
            ON r.RN + 1 = S.RN 

)

select  
    [year_month]
    ,[original_threshold]
    ,[change]
    ,[manualChange]
    ,[new_threshold]    
from cteR


output for it:

year_month original_threshold change      manualChange new_threshold
2017_01    458                0           0            458
2017_02    439                0           0            439
2017_03    412                0           0            412
2017_04    390                0           0            390
2017_05    393                -3          0            390
2017_06    389                0           -7           382
2017_07    397                -15         -5           377
2017_08    396                -19         0            377
2017_09    401                -24         0            377