Taxpayers are coping reasonably well in the first filing season under the 1986 Tax Reform Act, but it is time to call a halt to change, Internal Revenue Service Commissioner Lawrence B. Gibbs said. With the filing deadline looming Friday, Gibbs said about 60% of an expected 107 million returns had been received and refunds are running about $20 higher than last year. The number of refunds is down about 2% so far, partially attributable to a 1% drop in processing. Only 48% have itemized deductions, he said, compared with two-thirds in 1987. "One of the real problems for all of us is the constant change in tax laws--19 times in the last 23 years," he told reporters before addressing the Economic Club of Detroit. "Folks, that's just too much. We are saying to Congress, 'Just leave us alone.' "