Getting prior record

Group by UnitID, then add a shifted column to each subtable.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjQw0HU0VNJRMlSK1UHiGqFyjVG5JnCuE6peJ1TFYK4pKtcMwTVCVWyEarIRpmILpdhYAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [UnitID = _t, LeaseID = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"UnitID", type text}, {"LeaseID", Int64.Type}}),

//Group by UnitID
// then add Shifted Column to each sub table and re-expand
    #"Group UnitID" = Table.Group(#"Changed Type",{"UnitID"}, {
        {"Shifted Column", (t)=>
            Table.FromColumns(
                Table.ToColumns(t)
                    & {{null} & List.RemoveLastN(t[LeaseID],1)},{"UnitID","LeaseID","Prior Lease"}),
                type table[UnitID=text,LeaseID=Int64.Type, Prior Lease = Int64.Type]}
    }),
    #"Expanded Shifted Column" = Table.ExpandTableColumn(#"Group UnitID", "Shifted Column", {"LeaseID", "Prior Lease"})
in
    #"Expanded Shifted Column"

Results