Updating rows based on earlier row

PaulBoden , We can try to create a new calculated column based on logic provided by you

AdjustmentOutput =
VAR CurrentID = 'YourTable'[ID]
VAR CurrentDate = 'YourTable'[Date]
VAR CurrentCase = 'YourTable'[Case]
VAR CurrentFlag = 'YourTable'[Flag]
VAR CurrentLevel = 'YourTable'[Level]

RETURN
IF (
CurrentFlag = "Adj",
IF (
CurrentCase = "L",
"H",
IF (
CurrentCase = "M",
CurrentLevel,
BLANK()
)
),
IF (
'YourTable'[Date] >= CALCULATE(MIN('YourTable'[Date]), FILTER('YourTable', 'YourTable'[ID] = CurrentID && 'YourTable'[Flag] = "Adj" && 'YourTable'[Case] = "L")),
"H",
IF (
'YourTable'[Date] >= CALCULATE(MIN('YourTable'[Date]), FILTER('YourTable', 'YourTable'[ID] = CurrentID && 'YourTable'[Flag] = "Adj" && 'YourTable'[Case] = "M")),
CALCULATE(MAX('YourTable'[Level]), FILTER('YourTable', 'YourTable'[ID] = CurrentID && 'YourTable'[Flag] = "Adj" && 'YourTable'[Case] = "M")),
'YourTable'[Output]
)
)
)