How much does it cost to change the crafting profession? It depends on your level? I haven't found something official... Also do you lose the crafting level of the previous profession?
You don't lose your crafting level if you switch professions, and the cost to change is all dependent on the level you have in that profession.
More information can be found here : http://wiki.guildwars2.com/wiki/Crafting
To learn a craft, speak to the relevant master craftsman; they can also answer various questions about the craft, including how to obtain materials or what can be produced. Although a character can only have two disciplines active at a time, they can pay a master craftsman to switch, at cost of 10 copper per level in the target discipline. Taking on a new discipline is always free, while switching back to a maxed discipline (at Level 400) costs 40 silver .