Combine multiple rows into new columns based on two columns

try this code to see how using Text.Combine and Split to achieve your objective. 

This works on the assumption that you have two lines of entry for each assignment.

You referred to using min. You can consider ranking and retaining top two records for each group as your change record.

Please mark resolved if it works for you.

let
Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjTXNbDUNTIwMlGwsDI1sDIxUXD0VdJRCvB09jY0NDA0NgZyHIuLM9PzclPzShRCMnNTgSKGJlZGBlYGhnqmhiAFBhC9RnoGFsZQzUqxOmSbDtIA0mhohswxh5scCwA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Date Time" = _t, docNo = _t, fieldCaption = _t, oldValue = _t, newValue = _t, userID = _t]),
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Date Time", type datetime}, {"docNo", type text}, {"fieldCaption", type text}, {"oldValue", type text}, {"newValue", type text}, {"userID", type text}}),
#"Grouped Rows" = Table.Group(#"Changed Type", {"Date Time", "docNo", "fieldCaption", "userID"}, {{"DATA", each _, type table [Date Time=nullable datetime, docNo=nullable text, fieldCaption=nullable text, oldValue=nullable text, newValue=nullable text, userID=nullable text]}}),
#"Added Custom List oldValue" = Table.AddColumn(#"Grouped Rows", "oldValue", each Text.Combine([DATA][oldValue], "|")),
#"Added Custom List newValue" = Table.AddColumn(#"Added Custom List oldValue", "newValue", each Text.Combine([DATA][newValue], "|")),
#"Split Column by Delimiter" = Table.SplitColumn(#"Added Custom List newValue", "oldValue", Splitter.SplitTextByDelimiter("|", QuoteStyle.Csv), {"oldValue.1", "oldValue.2"}),
#"Changed Type1 delimiter oldValue" = Table.TransformColumnTypes(#"Split Column by Delimiter",{{"oldValue.1", type time}, {"oldValue.2", type date}}),
#"Split Column by Delimiter1" = Table.SplitColumn(#"Changed Type1 delimiter oldValue", "newValue", Splitter.SplitTextByDelimiter("|", QuoteStyle.Csv), {"newValue.1", "newValue.2"}),
#"Changed Type2" = Table.TransformColumnTypes(#"Split Column by Delimiter1",{{"newValue.1", type time}, {"newValue.2", type date}}),
#"Renamed Columns" = Table.RenameColumns(#"Changed Type2",{{"oldValue.1", "Assignment Time oldValue"}, {"oldValue.2", "Assignment Date oldValue"}, {"newValue.1", "Assignment Time newValue"}, {"newValue.2", "Assignment Date newValue"}})
in
#"Renamed Columns"