Now, if my H1b is approved before the start date, I have to go back China and apply H1b at US embassy, right?
-Not need if the H1 approval has the I-94 and your old H1 I-94 does not expires, then you can stay in US and wait for the H1 start date to work again. If your old H1 I-94 has expired before the new H1 start date and the H1 transfer is also approved before the start date, yopu have to go back to China to apply for the H1 visa to re-enter US in advance up to 10 days before the new H1 start/effective date and then, work on the H1 start date.
In this case, will my current H1b start date still be valid? (which means I can go back US on the start date after getting visa approved in China)
-Yes. You just show your new H1 approval notice to the IO at the port of entry and the IO will give you the new H1 I-94 to match the new H1 approval notice duration.
