Necessity of rebuilding pkgsrc after updating the system

[Pongthep Kulkrisada <ptkrisada%gmail.com@localhost>]

Hi all,

After updating the system from 6.x to 7.x (i.e. update across major branches),
should I have to rebuild all pkgsrc?
If yes, how to force rebuild it?
I looked through the handbook, I didn't find the information.
# pkg_chk -u -s doesn't do the trick with the same quarter of pkgsrc.

Thanks in advance,

-- 
Pongthep Kulkrisada
 
"Nothing ever exists entirely alone without relativity."