This question will mostly deal with your everyday soldier as many former KMT divisions were left behind due to a lack of transportation available. There were also a few notable cases of betrayal and capture between 1947-1949 that left some higher ranking officers and governors in the hands of the communists.

Soldiers

The KMT army at the end of the civil war was roughly 4-5 million strong, and our estimates are that about 2 million were successfully transported to Taiwan, so anywhere from half or less. Mao ordered his generals to incorporate these troops into the PLA, which was overall a successful program. Naturally, it can be assumed that a fair amount of these troops also just fled back to their home villages if given the opportunity, but we're not sure how many. The former Nationalist troops provided new armies to bulk the already large and experienced PLA, but the PLA's purpose in 1949 wasn't entirely clear until the outbreak of the Korean War.

The Korean War provided Mao with one of his greatest political opportunities that would make any dictator jealous. With Korea evolving into a more and more destructive environment, Mao saw his chance to rid the CCP of potential political enemies and former Nationalist troops who may or may not cause future problems. When China deployed to Korea, the new PLA armies composed of ex-KMT troops, like the 50th PLA Army, saw a lot of front-line action, and as a result massive casualties compared to other PLA armies. After the war, assuming that they stayed in the PLA, they were generally assigned to fringe areas across China, like Tibet. The 149th division, now the 52nd, was a division within the 50th army and in contemporary times acts as a motorized mountain infantry division on China's border with India.

Higher Officials

There were some notable KMT betrayals throughout the Civil War, as well as some generals surrendering and then being held captive for some time. Tang Shengzhi, one of the KMT's more popular generals, surrendered to the CCP in Hunan, where he agreed to become a local administrator. Probably the most famous KMT general to be captured was Du Yuming, the general in control of the Northeast KMT divisions who scored some early victories against the communists, but was relieved of duty in 1947. Du was forced to take a government role after the CCP victory. Fu Zuoyi, general of the forces around Beijing, surrendered to the CCP early on as well, taking a government position in the PRC.

While they often lived in comfort throughout the 1950s, once the Cultural Revolution started up many of these former KMT politicians were the first to suffer the wrath of Red Guards. Fu Zuoyi's daughter was humiliated several times during the CR, for example. Anyone who had any past connection to the KMT, or education in the West, was a primary target for Red Guards, resulting in their being purged, perhaps rehabilitated, and then possibly purged again, which also happened to a fair amount of life-long CCP members.

Finally, in the 1960s, Zhou Enlai announced a campaign to try and convince notable overseas Chinese individuals to return home and help the mainland. The most notable ex-KMT official who returned was Li Zongren, one-time president of the KMT and a very powerful general during WW2. Li's reasons for returning are unknown officially, but historians have long noted that Li and Chiang despised each other; their first falling out was in 1929, and they would have to share power for decades after that. Li was treated like a hero, mainly being used for propaganda purposes by the CCP, but ultimately died just a few years after returning of cancer.