SQL Server - swap or update 1st column value into 2nd column values

Hi All,
 
I have a table with data like something as -
 
id1    id2
1        5
2        8
3        6
 

Output like -

id1        id2
5            1        
8            2        
6           3       

I want to swap value all values of 1st column in 2nd column and 2 columns value into 1st column.

Put up a test table and try this against test data


update mytable
   set cola = colB,
     colb = cola
where some key = blah

I "think" this should work because the commit on the transaction should not happen after the set cola = colb, it should happen after everything is done.