The error message is in red color 'You are entitled to have one / two free Regional Packs'. There's no option to select anything other than base pack in that page.
1) as my guess says in any case change of base pack has 1 day as lock in period so after any 1 change in base pack you have to wait 24 hours for next change.
2) mytatasky.com web can only do upgrades that is lower rate pack to higher rate pack changes. to downgrade that is higher value pack to lower value pack you have only one way that is either chat with cc over phone or chat with cc over chat option in helpdesk of mytatasky.com. your pack is ultra with i guess 560 rs per month so even if you try to bumper with 425 as both having same 2 free regional pack you will be rejected.
note(for other users) : mytatasky.com will be able to add or drop addon, alacarte channels and services but not in the case of base pack. base pack can only be upgraded. to downgrade base pack you 100% will have to contact cc.