Hello All ,
I recently came across to a weird situation in which i had to modify the existing channel availability plan timings for PI 7.31 single java stack.
In our project we have multiple availability plans for different channels.
Problem :
1. While going to channel plan SAP PI does not give the option to edit the planning straightaway.
2. there is no option to modify/edit the existing plan.
3. In absence of this i had to create a new availability plan and delete the old one,which was time consuming.
Solution :
1. Go to options and click on to create a New availability plan.
2. Immediately after this delete this plan without making any changes in to it.
3. After this step all your existing available plans will show the modify section and you can go ahead modify them and save them.
This is bit strange but hope this helps 
.
Thanks
Gaurav Ranjan