First Union Corp. said it wrapped up its conversion of the former CoreStates Financial Corp. last weekend.

The conversion of three million customers' accounts began Nov. 5 and was completed Saturday. Executives of the Charlotte, N.C., banking company congratulated themselves on a job well done.

"This to us is a historic conversion because it was the largest and most complex we've ever done," said James A. Chester, First Union's senior vice president of mergers and acquisitions. "On a scale of one to 10, I'd give it a nine-something. Nothing is ever a 10. Not something this big, anyway."

The relative ease with which First Union converted the more than 400 systems run by Philadelphia-based CoreStates was in sharp contrast to the company's experience when it converted the much smaller Signet Banking Corp. of Richmond, Va., this year.

And it came under the watchful eyes of many competitors who have been working to attract customers and employees from the former CoreStates while First Union spent the last several months trimming jobs and branches.

But First Union officials, weary from the many public relations problems that have plagued the merger, said they worked overtime to avert conversion-related problems.

In addition to flying in more than 1,000 First Union employees to help out in the branches and in an employee call center, the company adopted several new conversion strategies.

For example, First Union decided to let CoreStates customers continue to use their old automated teller machine cards after the conversion.

In previous mergers, the company had issued customers new ATM cards and deactivated their old ones.

But under the latter system, some customers had trouble getting access to their funds.

Keeping the 1.2 million CoreStates ATM cards activated is a major undertaking because of the effort and expense required to maintain the old account data and combine it with the new systems, said Mr. Chester.

But "the absence of ATM problems is evidence of the value of the strategy," he said.

First Union will continue to honor the CoreStates cards until customer activity indicates transactions on the old cards have shrunk to less than 5% of the CoreStates customer base.

To avert another potential problem, First Union put signs on hundreds of ATMs and sent letters and statement stuffers notifying customers of a small change in the routine for gaining ATM access to money market accounts.

At CoreStates, they had to hit the savings-account prompt to get into their money market account; now they must hit the one for their checking account.

"It's the little things that come back and bite you," said Mr. Chester. "We watched out for those."

Also, in mid-October, First Union for the first time did a network readiness test to simulate the conversion.

Four hundred CoreStates branches made transactions that were run through First Union's systems.

The company also simulated call volumes to its voice response units, measuring response times and looking for bottlenecks. Some problems were identified and corrected before the real conversion, Mr. Chester said.

Still, First Union officials don't deny there were a few bumps in the road. Customers faced long lines at many branches during the 10-day conversion, and at one point the entire First Union branch network had to deal with a 45-minute computer glitch that forced tellers to work off-line.

But overall, things went well-so well that First Union let workers take Veterans Day off instead of taking additional training, said Kim P. Hocutt, vice president of merger support for First Union.

About 75 extra workers are being kept in the former CoreStates branches to help out with heavy traffic, said Ms. Hocutt. They are likely to stay there till mid-December.

Other than that, "it's now business as usual," she said.

John B. Moore Jr., an analyst at Interstate/Johnson Lane, said First Union's previous merger experience helped it engineer the smooth conversion.

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