Prolong tourist visa

Hi. I have a tourist (L) for two entries. I came to China on Feb 19th. I leave it (go to HK), then come back on March 17-18th. If I apply for a new on Apr 9th, let's say, do I still need to leave the country in April then? Or it'll be enough of the fact that new one cancels the old one? I don't intend to overstay or violate the laws, also don't want trouble with future visas.
Thanks in advance!