My H1B petition(I797A) and H1B Visa are valid until 01/18/2019. I recently got my H1B extension petition approved with the validity from 01/19/2019 to 12/27/2021. I am planning to get my Visa stamped in India so that I get my H1B visa until 12/27/2021 in the passport. if the stamping gets done successfully and i get visa stamped till 12/2021, what will happen to the current unexpired visa?. would there be a case where they cancel out my current unexpired visa which is valid until 18th jan 2019 and i would end up in a situation where i may not be able to travel to US until 19th jan 2019 which is the start date of my latest petition. Please clarify and suggest accordingly.