Approved i-140 from firm a, moved to firm b, now want to switch to firm c.

[amjad163]

i have approved i-140 *from firm A, been with firm B for past 1 year and they have not filed labor yet. i have h1b until Dec 2019. so is it ok to switch to firm C now in the next 3-6 months? i will still be left with about*18*months h1b when i move to firm C. i suppose i need to have firm C labor approved by Dec 2019 to get another extension? any other complications to watch out for?