The way they could of done it, would of been to add some npc in lags arch, where those that had already paid for a pick from the gem store, could then hand it in to upgrade to the new one. Whether for a small gem price or whatever.
Of course, the best & only way they should of done it, would`ve been to not start selling things on the gem store that drop items, that normal in game bought picks do not.
They managed to survive with Guild Wars without pulling this stunt for over eight years to date.
If they`re desperate for money, they should start adding decent things to the gem store, like getting rid of the virtually pointless town clothes, & start doing more armour you can use in combat. Add more storage spaces etc