Switch column value selecting from slicer

You can do that by creating a field parameter for each of the sets of measures in your slicers.

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

Go to Modeling > New parameter > Fields

Name the parmeter and add the measures you want in that selection.

Add the paramater to a slicer and to to your visual.  I would set the slicer to drop down single select.  When you change the measure selected in the slicer the column name changes in the table.

Sure, this is going to look more complicated that it really is probably.  There is a bug? with field parameters where you cannot use SELECTEDVALUE to read the selection.  Here is an article on it: https://www.sqlbi.com/blog/marco/2022/06/11/using-selectedvalue-with-fields-parameters-in-power-bi/

That means we have to use the longer SELECTCOLUMNS SUMMARIZE route shown in that article. 

We use that to see what is selected in the Reference Scenario and Comparison Scenario 1 then use SWITCH to get those amounts and finally do the compare.

YTD Ref - Comp 1 = 
VAR _RefSelection =
    SELECTCOLUMNS (
        SUMMARIZE (
            'Reference Scenario',
            'Reference Scenario'[Reference Scenario],
            'Reference Scenario'[Reference Scenario Fields]
        ),
        'Reference Scenario'[Reference Scenario]
    )
VAR _CompSelection =
    SELECTCOLUMNS (
        SUMMARIZE (
            'Comparison Scenario 1',
            'Comparison Scenario 1'[Comparison Scenario 1],
            'Comparison Scenario 1'[Comparison Scenario 1 Fields]
        ),
        'Comparison Scenario 1'[Comparison Scenario 1]
    )
VAR _Ref =
    SWITCH (
        _RefSelection,
        "YTD ACT", [YTD ACT],
        "YTD FCST (6+6)", [YTD FCST (6+6)],
        "YTD FCST (9+3)", [YTD FCST (9+3)],
        "YTD FCST (3+9)", [YTD FCST (3+9)],
        "YTD BDG", [YTD BDG]
    )
VAR _Comp =
    SWITCH (
        _CompSelection,
        "YTD ACT", [YTD ACT],
        "YTD FCST (6+6)", [YTD FCST (6+6)],
        "YTD FCST (9+3)", [YTD FCST (9+3)],
        "YTD FCST (3+9)", [YTD FCST (3+9)],
        "YTD BDG", [YTD BDG]
    )
RETURN
    _Ref - _Comp

Please note, it will look like the DAX is throwing an error, even though it works fine: