alter table modify primary index to unique primary index - response (6) by mmongia08

HI Dieter,
 
Could you please explain on your comment. As if in any case we have to have the table empty then there is no use of these workarounds. Please confirm the steps.
I have created the table with a Non unique primary index and then i inserted one row. Afterwards i defined one secondary index(USI) then the alter command to change it to Unique primary index. It gives me error that the table has some data