Dynamic Field headings for matrix reports

WhyNotLogical,

Here's a solution that uses field parameters (see link below). Create a field parameter for each version. In your model, you would have six rows (one for each measure) in each field parameter.

VersionV1 = {
    ("Actual", NAMEOF('Measure'[Actual]), 0),
    ("Budget", NAMEOF('Measure'[Budget]), 1),
    ("Forecast", NAMEOF('Measure'[Forecast]), 2)
}

VersionV2 = {
    ("Actual", NAMEOF('Measure'[Actual]), 0),
    ("Budget", NAMEOF('Measure'[Budget]), 1),
    ("Forecast", NAMEOF('Measure'[Forecast]), 2)
}

Create a variance measure:

Variance = 
VAR vVersion1 =
    TREATAS ( { MAX ( VersionV1[VersionV1] ) }, FactTable[Version] )
VAR vVersion2 =
    TREATAS ( { MAX ( VersionV2[VersionV2] ) }, FactTable[Version] )
VAR vVersion1Amount =
    CALCULATE ( [Amount], vVersion1 )
VAR vVersion2Amount =
    CALCULATE ( [Amount], vVersion2 )
VAR vResult = vVersion1Amount - vVersion2Amount
RETURN
    vResult

The Amount measure is a sum of your fact table amount:

Amount = SUM ( FactTable[Amount] )

 Create a matrix with the field parameters and Variance measure:

Result:

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https://learn.microsoft.com/en-us/power-bi/create-reports/power-bi-field-parameters