I had almost the same experience as you early this year. They took away my original F visa I-94 and gave me a new I-94 with H-1 status. Xiaobaitu is correct. You are still in H-1 status even you first came to US with a F-1 visa. The original visa type does not matter. I had done extensive researches and consulted with more than 2 laywers on it.
The only difference is I had not submitted my I-485 at that time, only with I-140 approval.