See if you want your caption to be dynamically changed then you need to use caption class.
If you just want to put a different caption to your field name then you can do it on the field property and in caption or else in page designer can then in caption.
Look into CaptionClass 3. This caption class let's you specify the actual string to be used. E.g.
CaptionClass='3,' + Rec2.FIELDCAPTION(SomeField)
This way you don't need to modify Codeunit 1 (or Codeunit 42 in newer versions of NAV).
The value of the property is a C/AL expression. You may call your own function.
Answers
You should look into the 'CaptionClass' functionality. You will definitely need a developer license for this, since a part of this is in codeunit 1.
If you just want to put a different caption to your field name then you can do it on the field property and in caption or else in page designer can then in caption.
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The value of the property is a C/AL expression. You may call your own function.