If you change your visa, now, you have to apply for a new F visa again in the Philippines. If you apply for extension after entering China, once your apply for visa extension, in fact, you will be issued a new F visa with more stay duration, but your multiple entry will become 0 entry. It is not advisable to change your visa, you can stay for 30 days, then go to Hong Kong/Macau, and return to Mainland China for another 30 days.