Oracle SQL:marcar registros según la fecha del registro frente al historial

Puede usar LAG función:

SELECT
"REGION"
, "COUNTRY"
, "IDNum"
, "CUSTOMER"
, "BUSSINESS"
, "FISCALYEARMONTH"
, "FISCALYEAR"
, SUM("VALUE") 
, MAX(CASE WHEN to_date(prev_fym,'YYYYMM')  >= ADD_MONTHS (to_date("FISCALYEARMONTH",'YYYYMM'), -24) THEN 'Existing' 
           ELSE NULL END) "HISTORICAL_PURCHASE_FLAG"
FROM
(
    SELECT
    T1."REGION"
    , T1."COUNTRY"
    , T1."IDNum"
    , T1."CUSTOMER"
    , T1."BUSSINESS"
    , T3."FISCALYEARMONTH"
    , T3."FISCALYEAR"
    , T4."VALUE"
    , LAG ("FISCALYEARMONTH", 1) OVER (PARTITION BY T1."IDNum" ORDER BY T3."FISCALYEARMONTH" DESC) prev_fym 
    FROM
    "DATABASE"."SALES" T4
    , "DATABASE"."CUSTOMER" T1
    , "DATABASE"."PRODUCT" T2
    , "DATABASE"."TIME" T3
    WHERE
    T4."CUSTOMERID" = T1."CUSTOMERID"
    AND T4."PRODUCTID" = T2."PRODUCTID"
    AND T4."DATEID" = T3."DATEID"
    AND T1."COUNTRY" IN ('ENGLAND', 'France')
    AND T3."FISCALYEAR" IN ('2014','2015','2016')
)
WHERE "FISCALYEAR" IN ('2016')
GROUP BY
"REGION"
, "COUNTRY"
, "IDNum"
, "CUSTOMER"
, "BUSSINESS"
, "FISCALYEARMONTH"
, "FISCALYEAR"
;