H1b Transfer with Extension Question

[shijocj]

I am having 1 year left for my current validity period and looking for opportunities. I am already past 6 year and also have I-140 approved. Since if I join new company I need to restart my GC from Perm wanted to know I will get more than 1 - 1.5 year of H1b validity if the new employer can submit enough documents to support me for 2-3 years, like client letter or project contract.

Can anyone tell what is the possibility for this and is there any conditions for this?