OffTopic : sed : Alternate (2,4,6..) replacement

Hi List
I need some help on this;
How do i replace the 2nd,4th,6th occurances of a column (vertical) in a file.
I know sed does it within a row

$ cat swt

01:this is a file1 this
02:this is a file1 this
03:this is line3 
04:this is line4

$ sed 's/this/that/2' swt

01:this is a file1 that
02:this is a file1 that
03:this is line3 
04:this is line4

01:that is a file1 this
02:that is a file1 this
03:that is line3 
04:that is line4

But i want

01:this is a file1 this
02:that is a file1 this
03:this is line3 
04:that is line4

I tried awk but awk seems to be going one line at a time.

Thanks in Advance.

GovindanK

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