Overwrite columns with logic

Hello Anonymous 

add a new column like this. The variable ChangeType has to reference to your prior step.

try if [#"Exit          "]="End" then "End" else if ChangedType{[#"Time interval "= [#"Time interval "]-1]}[#"Exit          "]="End" then "Commence" else null otherwise null

 Here a complete example

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUXLOz81NzUtOBTLzSnNykEVidaKVjBDirnkpUBIkYUxYrwlCHJkCSZniljLDLWWOyzEWuPVYEnanoQFu7YaGeORwBo6hMR5dJkS4CFf4xAIA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Time interval " = _t, #"Enter               " = _t, #"Exit          " = _t, #"Enter/Exit" = _t]),
    ChangedType = Table.TransformColumnTypes(Source,{{"Time interval ", Int64.Type}, {"Enter               ", type text}, {"Exit          ", type text}, {"Enter/Exit", type text}}),
    #"Added Custom" = Table.AddColumn(ChangedType, "Custom", each try if [#"Exit          "]="End" then "End" else if ChangedType{[#"Time interval "= [#"Time interval "]-1]}[#"Exit          "]="End" then "Commence" else null otherwise null)
in
    #"Added Custom"

Copy paste this code to the advanced editor in a new blank query to see how the solution works.

If this post helps or solves your problem, please mark it as solution (to help other users find useful content and to acknowledge the work of users that helped you)
Kudoes are nice too

Have fun

Jimmy

Anonymous  can you please try this

let
    Source = Web.BrowserContents("https://community.powerbi.com/t5/Power-Query/Overwrite-columns-with-logic/m-p/1637835#M50128"),
    #"Extracted Table From Html" = Html.Table(Source, {{"Column1", "TABLE:nth-child(5) > * > TR > :nth-child(1)"}, {"Column2", "TABLE:nth-child(5) > * > TR > :nth-child(2)"}, {"Column3", "TABLE:nth-child(5) > * > TR > :nth-child(3)"}, {"Column4", "TABLE:nth-child(5) > * > TR > :nth-child(4)"}}, [RowSelector="TABLE:nth-child(5) > * > TR"]),
    #"Promoted Headers" = Table.PromoteHeaders(#"Extracted Table From Html", [PromoteAllScalars=true]),
    T_1 = Table.TransformColumnTypes(#"Promoted Headers",{{"Time interval", Int64.Type}, {"Enter", type text}, {"Exit", type text}, {"Enter/Exit", type text}}),
    L= T_1 [#"Enter/Exit"],
    Terminate = List.Count(L),
    Loop = List.Generate(
                          ()=>
                              [i=0, currentItem =L{i}, nextItem=try L{i-1} otherwise null ],
                              each [i]<Terminate,
                              each [i=[i]+1, currentItem=L{[i]+1},nextItem=try L{i-1} otherwise null],
                              each [nextItem]
                              
    ),
    name = Table.ColumnNames(T_1),
    val = Table.ToColumns(T_1),
    newName=  List.Combine({name,{"newVal"}}),
    newT= Table.FromColumns(val&{Loop},newName),
    #"Added Custom" = Table.AddColumn(newT, "finalVal", each if ([#"Enter/Exit"]="null" and [newVal]="End") then "Commence" else if ([#"Enter/Exit"]="Commence" and [newVal]="null") then "null"else [#"Enter/Exit"]),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"newVal"})
in
    #"Removed Columns"