You don't necessarily have to change the disc every time you change the pads. It really depends on the condition of your disc. If you touch it with your fingers, you can see how much it has used up. Some people change their disc every 2-3 times changing their pads. Personally for me, I never skim my disc and I will change only if necessary. As for the Brembo OEM products, I suggest you to be very careful with them. I've used a Brembo MAX rotor (so called Brembo OEM) but it's actually some bull shit product. It squeaks badly, the rotor doesn't use up evenly and all these suggest to me as if they are some China made imitation. If you want a good pad, I have an extra box of Stop Tech brake pads which I've ordered directly from USA. Genuine product and priced slightly more than the original pads you can buy locally but the Stop Tech gives a better bite/braking power.
On my ride, I have upgraded to Stop Tech cross drilled + slotted rotors in the front and Stop Tech brake lines, makes a hell lot of difference.