If working in a salaried position and being in sales (during a time of low sales) can your employer "pull salary" away from you and place you solely on commission effective immediately, even if there is an employment contract signed stating the annual salary?
Your employer cannot unilaterally change your compensation structure in a severe way. There are many cases that state going from salary to commission is a real and substantial change to your compensation such that you can consider this action as amounting to your dismissal. In other words, you can reject the change and look for other work or you may be able to simply leave and claim you were dismissed. You could then sue for damages for your economic losses while you are out of work.
As this area of the law is quite complex and your election is important, I would recommend meeting with an employment lawyer who can explain your rights to you in greater detail and build a strategy that best fits your particular situation.