If the symptoms occur just when changing strings, chances are very good that this is just a neck adjustment that is needed. Heavier strings pull the neck up more because they have more tension. I bet the luthier did the set up based on the string guage you were using. The sound you are hearing on the A string is from the string rattling on the first fret. This is because there is too little relief in the neck when you switch to light strings. They have less "pull" or tension than the heavier strings you had.
There are really only three things that can cause action and setup to change. First--the weather. Wood changes with relative humidity and temperature. Second, changing string guages to either lighter or heavier than what was used when the instrument was set up. Third, and less likely, the truss rod is not holding its tension--but that is much less frequent.
Decide what guage and type of strings you really want, go back to the luthier and explain whats happening. A little tweak of the neck and your problems will be solved. Don't start messing with the nut. Its very possible that the nut is not the problem. You got a real fine hot rod setup probably, but it requires the same guage of strings you started with. Once you start changing things, the action and setup will change too.
Wisdom comes with age, but sometimes age comes alone
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