I'm not sure what the discount is with a PIN code but if a member discount is available at that time it would be 25% off rack rate. When we were planning our May trip we found that the 25% off of the studio at VWL was only about $60-80 more than a AP rate room at WL. I don't know how the PIN code discounts compare to the AP rates. For us, it was worth just staying with the studio at the member discounted rate. I think the best way to find out is call MS.
Another thought, if you do a cash night through MS, they might be able to link your reservation so that maybe you won't have to change rooms after the one night (assuming you pay cash for the same size room).