To reset an arch skill and allow you to choose another ...
The the skill can be reset back to zero and
a penalty of 100k exp is applied. You do not need to have 100k experience to lose it, you can go negative for a short period.
To reset a profession to allow you to choose another ...
Reduce profession skill to zero
Remove 100k exp (same as for arch skills)
If Blacksmith reduce profession rank to 50% (one half)
If Craft reduce profession rank to 33% (one third)
If Harvest or Alchemist reduce profession rank to 25% (one quarter)
Skill reduction is based on perceived difficulty in raising it..