#I have tried the following commands:
$cat numbers
1       2
10      20
100     200
1000    2000
10000   20000
100000  300000
1000000 3000000

cat numbers | cut -f 2 > copynumbers

#How do I get field 1 from the original file into field 2 of the output file?
$cat copynumbers
2
20
200
2000
20000
300000
3000000

$ paste <(cut -f2 numbers) <(cut -f1 numbers)
2        1
20       10
200      100
2000     1000
20000    10000
300000   100000
3000000  1000000