NO MEASURE of Mideast change is more telling than the fact that Israeli Prime Minister Menahem Begin, who is now in Washington, is evidently presenting to President Carter the first fruits of his "rethinking" on the West Bank. This is, of course, the territory that in the past he ardently claimed for Israel on both strategic and religious grounds. That he has so promptly and interestingly opened up his position is the most solid kind of evidence that he and Egyptian President Anwar Sadat are not headed straight for a "selfish" separate peace and, indeed, that Mr. Sadat's Jerusalem gamble is having just the transforming effect that he hoped it would. We add that Mr. Beginis also presenting certain ideas tying further Israeli withdrawal in the Sinai to new and mutual security arrangements there. These ideas are no less significant for being less eye-opening.

The Begin government's revised West Bank (and Gaza) approach or, to be more exact, its current bargaining position or its position on the first phase of a peace, is known so far only through leaks in the Israeli press. It seems that Israel would end its military occupation and hand the territory to an Arab civilian administration. Neither Israel's nor any other nation's sovereignty would hold sway, but there would be "a certain kind of link" with Jordan. Israel would maintain a security presence. As fulfillment of what Mr. Begin feels to be Israel's unbreakable biblical tie to the land, there would be no legal restrictions on further Jewish settlement. Israeli citizenship would be available to those residents seeking it. Mr. Begin now calls them, by the way, "Palestinian Arabs," a term apparently chosen to identify them as a group whose interests must be considered but to deny them the status and the putative claim on statehood that the word "Palestinians" conveys.

The point is: The process of negotiations is under way.That the Cairo conference has begun and that Mr. Begin an Mr. Sadat may meet again, even this month, further show their intent to move the process along. As hopeful as we are, we are no better able than anyone else to predict the outcome.

We are, however, persuaded of one thing, and that is that there need be no further contentious argument over what the American role is. If earlier the administration had some difficulty accepting that its own center-stage prominence as goad and mediator was no longer necessary, then it now seems more aware of the dimensions and demands of its new supportive role. That role requires Washington not to squeeze concessions out of Israel or to press Egypt to accept hard-to-stomach Israeli demands, but to get the best sense it can of the process of the negotiations, to assist where the parties agree that it can be helpful, and to do nothing to hinder the bargaining.