Replacing A Column Of Numbers

Hello,

New user of Excel 2007, and Excel in general.
Should be simple, but I just can't figure out how.

Have a column of numbers in Column A
In Column B (row 1)I have the formula of what's in A (row 1) multiplied
by 1000

And I have dragged this formula downwards, so I now have in each row B
what is in the same row in Column A multiplied by 1000.

So far, all is fine.

Now, I want to replace what's in Column A by the newly generated values
in Column B (row by row replacement)

I can't just Cut out everything in Column A, as B then disappears, as,
I've finally figured out, Column B values depends on Column A values.

How do I move everything I've generated in Column B to Replace what's in
Column A ?

Much thanks,
Bob