A MISCHIEVOUS game is being played by those in Washington and Taipei who speak of "upgrading relations" between the United States and Taiwan. Ostensibly, they mean only to ensure that the United States abides by the pledge it made after it normalized relations with China -- to provide, in the language of the Taiwan Relations Act, "adequate safeguards for Taiwan's security and well-being." And if this were all, no one could complain. On the contrary, given the feelings about Taiwan in this country, people would probably applaud.

But it isn't all. "Upgrading relations" has become a code phrase for something different. In Taiwan, where officials have just said they have received signals that the administration will upgrade relations, the phrase betrays an intent to make trouble between Washington and Peking. The officials are well aware that Washington, in normalizing relations with Peking, made relations with Taipei unofficial. "Upgrading" even symbolically -- by raising the number of Chinese "liaison offices" or making contacts between representatives more formal -- is seen as a way to erode normalization. Selling Taiwan super-hot new warplanes for which no feasible military case has been made could be similarly exacerbating.

In Washington, meanwhile, "upgrading" is at once a bow to Mr. Reagan's core belief and constituency and a thumb in Peking's eye, the better to give Peking to know that American cooperation is strictly business.

In fact, as most of the professional now accept, "upgrading relations" with Taiwan cuts across the American interest in both Peking and Taipei. To antagonize Peking gratuitously just as strategic cooperation is being deepened is a reckless inconsistency. Nothing could do more to close Peking's opening to Washington and to undercut its current friendly leadership than to act in a manner feeding charges that Deng Xiaoping has "sold out" on Taiwan.

The best way to make good on the American commitment to Taiwan is to hold firm in avuncular patronage and to encourage the island's moderate mainstream, which is coming to see its future in a context of peaceful cooperation with the mainland. Already there is trade worth hundreds of millions of dollars a year. It is a cruel joke, as well as a political error, to substitute a chimerical "upgrading of relations" for the real support that Taiwan's "security and well-being" deserve.