This from my (personally customized) help page:

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Go to System Mgr, TAS-D (Maintain Database).

Select BKPRMSTR.BVH
From the box that pops up, click the double-arrow to show all fields
At the top of the screen is a check-box that says "Allow editing". Put a check-mark in it.
The far left column has the employee numbers. Find the one(s) you want.
Select the box with the number and press <Enter>
Then change to the new number and press <Up> or <Down> arrow key.
It will ask you if you want to save the current record. (Yes).

Then we go to UT-K-B (Global field replace)

File Name is BKPRHIST
Scroll to and select BKPR.CURP.EMPNM as the Field Name
In "Enter value to replace" put the OLD employee number
In "Replace with value" put the NEW employee number
When it's done it will tell you how many it changed.
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You probably shoudl try it in your SandBox company before doing it for real.....