Hi Sandeep
In the definition of the agreement, transaction VB(2, there is field Different Val.Period V_T6B1-ABWDT. If you set that flag it allows different condition validty periods so they are detached from the agreement validity dates. If that flag is not set when you change the agreeemnt validity dates it would also trigger the update of the 20 conditions you have in this case.
The intention of the flag is to separate the two so you would have to make any condition changes
manually as you pointed out.
Hope this helps
Kind regards
Brian