Downsizing 300,000 to a million soldiers and expanding the PLAAF and PLAN will require an extensive period of reorganization
Ground forces are still comprised of 1.6 million soldiers. Many more than that, the previous cuts are not yet complete. As before most of them with their training and work experience will join the PAP (people's armed police, a paramilitary organization) who now number close to 1.5 million. They are internal border guards and a secondary standing reserve force. You see them standing around guarding public property in their green police uniforms and they don their black riot gear and white armored vehicles when quelling protests. They might also opt for the regular public police, who are unarmed but who are severely lacking. China only has ~2 million police officers, per capita that is completely inadequate. The Coast Guard (which belongs to PAP) is also being expanded dramatically. Another possible alternative.
So Beijing receives the dual benefit of streamlining/speeding up modernization of its armed forces, and also bolstering security domestically. There is absolutely no need for such a large standing force of professional troops. Conscription only exists on paper and has done so for decades now, and reserve duties take the guise of only 500,000 part time reservists and the full time PAP. At any one time there are ~5 million men with extensive military service experience that can be called up immediately, and a further 70 million potential conscripts that could undergo basic training. (Male and Female members of the Chinese communist party-a prerequisite for joining the armed forces in any capacity).
By shrinking the ground forces, and transferring them to cheaper to maintain PAP or to the civilian sector; they can afford to procure and maintain many more modern aircraft, warships, etc. The largest component of the military budget is maintaining salaries of full time soldiers.
This would transfer the strategic standing of the PLA entirely, from one of defense and focus on fighting a land war in Asia, to offense with focus on power projection abroad. It is growing apparent that China does not expect to be fighting any major ground conflicts on land. This is evident with the pan-Asian One Belt, One Road initiative. Instead, it is gearing up for limited high intensity conflicts in the Pacific, Indian Ocean, Africa, etc. This means having a credible deterrent that can safeguard its sea trade routes and east coast and also an air-force that would pose a major threat to any regional fleets or airbases.