They largely didn't, at least not in the Chinese definition of 'Tibet.' The Communists have liberated the serfs in the non-traditional Tibet regions where there are a lot of Tibetans, but because of the agreement with the Tibetan government have largely delayed the introduction of land reforms, thus you would in the early 50s saw the PLA commanders and local lords taking similar positions in regional councils. Mao has, strangely enough, largely kept his promise in the 17 Point Agreements.

So funny enough, today the comment would be about liberating the serfs, but back then, there wasn't a lot of serfs been liberated in the 'outer' Tibetan regions.

In fact, Mao had promised the following point

*In accordance with the policy towards nationalities laid down in the Common Programme of the Chinese People's Consultative Conference, the Tibetan people have the right of exercising national regional autonomy under the leadership of the Central People's Government. *

The central authorities will not alter the existing political system in Tibet. The central authorities also will not alter the established status, functions and powers of the Dalai Lama. Officials of various ranks shall hold office as usual.

The religious beliefs, customs, and habits of the Tibetan People shall be respected, and lama monasteries shall be protected. The Central Authorities will not effect a change in the income of the monasteries.

In matters related to various reforms in Tibet, there will be no compulsion on the part of the central authorities. The local government of Tibet should carry out reforms of its own accord, and when the people raise demands for reform, they shall be settled by means of consultation with the leading personnel of Tibet.

And this was largely respected in the outer Tibetan regions. So there were some grumblings from the PLA commanders who bitterly complain about their fighting went to gains of the major landowners in Tibet, but nevertheless, between 1951-1959, PLA forces largely kept out of local affairs and did not pursue the sort of land reforms in 'outer' Tibet. There were plans to began introduce more socialist reforms in 56, but Mao intervened and generally allow for life to go on, as usual, relatively speaking compared to rest of the China going through major land reforms.

And that's when the problem began for 'outer' Tibet. In 56-57, while Mao paused the plan to reform land use in Tibet, the rest of China went through major changes, some of which include territories where minorities lived. In some of these territories where the British would call 'inner' Tibet, land reform swept through Sichuan province and Qinghai, and Tibetan towns and villages went through the land reform all people went through, and major upheaval drove a lot of major landowners and man of means into 'outer' Tibet where they either form the key components of rebels or key supporters for the 59 uprisings. The group of rebels is mostly from the Kham region. With the likes of Gompo Tashi receiving aid from the CIA, they would clash not only with local PLA garrison but from time to time have to pressure or cajole local monasteries to support them, and quite often, even they would not be successful to convince their brothers to support them in this struggle. Tension has slowly built up from 57 to 58, where rebels would hit the PLA supply line, and the PLA would retaliate, and this would eventually led to the fatful days in 59, where Dalai openly renounce the 17 pt agreements, and the PLA began land reforms in Tibet.

So then the question to your answer would be, what was Mao's original goal? Was it to free peasants from the old feudal society? If it were, then he wasn't really doing it for 8 yrs, and has repeatedly promised to delay the reform for at least 2 terms [6 yrs a piece] if the situation isn't right. Though there probably are elements in the Chinese government that wanted to implement major land reforms, Mao was careful to not do so in the outer regions. But he certainly did not oppose any land reform in minority territory in the traditional provinces where serfs are liberated. So, it's a complicated topic, and my personal take is a no, the Chinese communist did not take over Tibet to free the peasants anymore than Lincoln went to war to free the slave. To Mao, and most of the PLA commanders, Tibet is a renegade region of the old China and has to be brought back to the fold, by diplomacy or by force, but by any means necessarily.

Source:

The Snow Lion and the Dragon: China, Tibet, and the Dalai Lama, by Goldstein

THE CIA's Secret War in Tibet, by Kenneth Conboy and James Morrison

In Exile from the Land of Snows, by John F. Avedon