L1A to H1B approved petition to company change

Hey all, I have an L1A stamped on my passport valid till 2020 and an approved H1B petition (not stamped) valid till 2021. I am now looking to change employers and was just asked by Microsoft if I plan to ‘activate’ (get H1B stamped on passport). Question: Have you been in a similar situation? Did you get H1B stamped before changing employers? AFAIK, this is not required and the new employer can simply file an H1B transfer petition under premium processing. Any experiences appreciated. ETA: TC 300k