Building a History Table from a Change Log and Current State Table

Hi OptiBill ,
yes, that's perfectly doable in PQ. This is the logic:

let
    Source = Excel.CurrentWorkbook(){[Name = "ChangeLog"]}[Content], 
    #"Changed Type" = Table.TransformColumnTypes(
        Source, 
        {
            {"Column1", type text}, 
            {"Created", type datetime}, 
            {"Field", type text}, 
            {"FromString", type text}, 
            {"ToString", type text}
        }
    ), 
    #"Removed Duplicates" = Table.Distinct(#"Changed Type", {"Field"}), 
    #"Removed Columns" = Table.RemoveColumns(#"Removed Duplicates", {"ToString", "Created"}), 
    #"Pivoted Column1" = Table.Pivot(
        #"Removed Columns", 
        List.Distinct(#"Removed Columns"[Field]), 
        "Field", 
        "FromString"
    ), 
    #"Merged Queries" = Table.NestedJoin(
        #"Pivoted Column1", 
        {"Column1"}, 
        CurrentState, 
        {"Key"}, 
        "CurrentState", 
        JoinKind.LeftOuter
    ), 
    _Initial = Table.ExpandTableColumn(#"Merged Queries", "CurrentState", {"CreatedDatetime"}, {"Created"}), 
    #"Pivoted Column" = #"Changed Type", 
    #"Removed Columns1" = Table.RemoveColumns(#"Pivoted Column", {"FromString"}), 
    _Ongoing = Table.Pivot(
        #"Removed Columns1", 
        List.Distinct(#"Removed Columns1"[Field]), 
        "Field", 
        "ToString"
    ), 
    Custom1 = _Initial & _Ongoing, 
    #"Filled Down" = Table.FillDown(Custom1, {"Assignee", "Status", "Priority"})
in
    #"Filled Down"

Please check out the file enclosed as well.