Testing code with Debug.Assert
The only thing that I can see at first glance to show what the business rule is, is to do something like this

m_GlobalCode = "ABC"
result = GetEquivalentCode
Debug.Assert result = "ABC2013"


probably at the beginning.  

I think this is exactly what the business rule is saying from my understanding of the information that you have given



you probably want to add something like this to follow it, just to be clear what the rule is.

m_GlobalCode = "AB"
result = GetEquivalentCode
Debug.Assert result = "AB"


These two pieces coupled together show the entire Business Rule.


  if the input string starts with "ABC" then return "ABC2013" otherwise return the input




6 tests should clearly define the Business Rule


Input: "ABC"
Input: "AB"
Input: vbNullString
Input: {Random String not Starting with "ABC"}
Input: "DABC"  Result: "DABC"
Input: "ABC5029DDD"


Number 5 shows that ABC must be at the beginning of the string

Numbers 1 &amp; 6 shows what the Function is doing.

Numbers 2,3,4, &amp; 5 show what happens when the ABC is not at the beginning