Upvote:3
No, you don't require transit visa for changing airport in USA, as transit visa is required on your way to a different country (with certain conditions and exemptions). Since, you already have valid B1/B2 visa for US and your travel destination is US itself so you don't need any transit visa for changing airport from 'Newark Liberty International Airport' to 'LaGuardia Airport'.
Your case is same as anyone who is visiting US as a tourist over B1/B2, he/she have to travel most of the parts of US as a tourist and that too mostly via Domestic flights, if transit visa was a requirement then every tourist have to get it for visiting US.