Replace values in up to 2 columns based on table values

Hi ktbb5757 ，
You can try the follwing code

let
    MainTable = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("LYw7CsNADAWvsqj2KfIjIaRJysXFWmhBWJaCVi58e29M2nkzL2c4OU/kGwxwXb5iG1F6Ms5TwRnGIcPZtApjJKvpoUFOLbr84oYkUpRsbYf4IVyd4/d0K9K4MpZg02O8vzu+9MZ5Yf3zcQc=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Prior Issue Type" = _t, #"Prior Subcategory" = _t]),
    IssueMapTable = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WcspMSi2qVNJRcs7PzU0tykxOzFGAisXqRCt5BAGlAlKLivPzgBJBqTmJJZn5ecUZmQXFCHm3xJzizDSgVpCcUmwsAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"New Issue Type" = _t, #"New Subcategory" = _t]),
    MainWithIndex = Table.AddIndexColumn(MainTable, "Index", 0, 1, Int64.Type),
    IssueMapWithIndex = Table.AddIndexColumn(IssueMapTable, "Index", 0, 1, Int64.Type),
    MergedTables = Table.NestedJoin(MainWithIndex, "Index", IssueMapWithIndex, "Index", "IssueMap", JoinKind.FullOuter),
    ExpandedTable = Table.ExpandTableColumn(MergedTables, "IssueMap", {"New Issue Type", "New Subcategory"}),
    ReplacedTable = Table.AddColumn(ExpandedTable, "Current Issue Type", each if [New Issue Type] = null then [Prior Issue Type] else [New Issue Type]),
    ReplacedTable2 = Table.AddColumn(ReplacedTable, "Current Subcategory", each if [New Subcategory] = null then [Prior Subcategory] else [New Subcategory]),
    FinalTable = Table.SelectColumns(ReplacedTable2, {"Current Issue Type", "Current Subcategory"})
in
    FinalTable

Final output

Best regards,
Albert He

If this post helps, then please consider Accept it as the solution to help the other members find it more quickly