Should MMOs change fundamental design rules post-launch?

A few weeks ago, ArenaNet added a new buyable perk to the Guild Wars 2 cash shop: a crafting license that allows an account's entire roster of characters to pick up an extra craft skill. In other words, buying this license means your characters can run three production craft skills, not just the original two at a time.

My first reaction was wooo, awesome! But after some reflection, I wondered whether the change wasn't more than cosmetic. Having three craft skills on every character affects leveling rates and PvE zone population, inflates the item market, impacts the resource market, and could even alter whether players buy other items from the cash shop (like character and inventory slots). It's definitely more than just a fun cosmetic thing. While this tweak probably won't matter that much in the long-run of a game like Guild Wars 2, fundamental changes made midstream have certainly been known to haunt some games forever.

What do you think -- should MMOs change core design rules post-launch? Where do you draw the line between a little quality-of-life tweak and a major shift in design philosophy?

TH