Right click the worksheet tab and select View Code
In the module that appears copy and paste the following code
Change the value of oRng (here ("A:A") to the column you wish to monitor.
Save the workbook as a macro enabled workbook.
Enter a number in the column
Code:
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
Dim oRng As Range
Set oRng = Range("A:A")
If Target.Cells.Count > 1 Or IsEmpty(Target) Then Exit Sub
If Not Intersect(Target, oRng) Is Nothing Then
If IsNumeric(Target) Then
On Error Resume Next
Application.EnableEvents = False
Target = "BC102014" & CStr(Target)
Application.EnableEvents = True
On Error GoTo 0
End If
End If
End Sub