Can I go from H4 to H1B (H1B > 6 years, have approved I -140 from previous employer)

[soumyaks]

I am on h1B for more than 6 years and have an approved I-140 from my previous employer. My current employer also filed for perm and currently my I-140 is pending for 2 months. My question is - if I move to h4, can I convert back to h1B later? What are the rules for converting from h4 to h1b.