Why truncate previous hints when upgrade from 1.1.9 to 1.2.6?

[Boole.Z.Guo (mis.cnsh04.Newegg) 41442]

Hi all,
When I upgrade C* from 1.1.9 to 1.2.6, I notice that the previous 
hintscolumnfamily would be directly truncated.
Can you tell me why ?
Because consistency is important to my services.


Best Regards,
Boole Guo