Yes. For the duration of your stay here on Res, you are allowed to switch classes 4times for FREE. However, if you still need to switch classes, you would have to purchase a Merchant Class Token or a Rancher Class Token depending on what class you wanted to switch to.
And yes, switching to Rancher will lose your discount at the NPC shops also your access to your Merchant Shops, however, the Rancher Shop will be available for you to sell pets.