2.then you need to change your H1 status to L1 again as you do now or go outside US to get the L1 visa to reenter US.
does it mean that i could change the status to L1 in US and then get the new L1 visa when i return to china for next time. and same situation could be applied to H4 visa, right?
-Correct.
3. if L1 visa is approved, am i eligible for EB1C application? any special requirments for application?
-Your L1 status does not mean you are qualified for EB1C. There are more requirements for EB1C.
