The welfare bill passed by Congress last week will eventually bring sweeping change to the nation's poor, but, in most states, it will probably not cause any sudden disruption. A little-noticed provision allows these states to continue operating the welfare programs they now have in place.

In theory, most provisions of the bill will take effect from Oct. 1 of this year to July 1 of next year. But under the bill, the 43 states that have received Federal waivers to run experimental welfare programs may continue to operate them, regardless of whether they comply with the new requirements.

The bill may ''end welfare as we know it,'' in President Clinton's words, but it may be years before the new welfare system takes shape.