1. I should be able to display when their subscription will expire (The subscription expiry date should be displayed) in the subscription page.

It already does this. See your Membership tab here at Joomlapolis and you'll see CBSubs showing when you expire. Example as follows.

"1 year Active, expiring on 2014-01-24"

This of course won't display for Donation, Merchandise, or Lifetime Subscription plans as they don't have an expiration.

2. Ability for the user to change the AutoRenew option (By default it is Auto Renew - On) using a simple link or script.

There's no feature for this. If they want to cancel they need to unsubscribe. If they want to go from single payments to recurring payments they can Renew and do so using subscription payment gateway button instead of single payment.

3. The user should be able to change the duration of subscription. By default it is one month. The user should be able to click on the Add Month button/link (as shown in the picture) and can increase the duration of month for his subscription.

This feature does not exist at all in CBSubs. Only way to have something like this is to copy your plan and setup multiple copies with different price and durations.

(i) How this can be done. Can it be done by implementing custom code.

For the most part it can't because some of the features you want don't exist. You can of course modify core code to do whatever you like, but we can't help you with this as we don't support core edits. Also note code edits voids support; meaning if you edit core files and it stops working we won't be able to help you until the edits are removed.

(ii) If so which are the files in CbSubscription (PHP and HTML/CSS) i should make code level changes to implement these use cases.

You can find CBSubs template files at the below location where you can freely adjust XML, CSS, and some HTML to alter the appearance, but this won't help implement some of the features you want that don't exist.

components/com_comprofiler/plugin/user/plug_cbpaidsubscriptions/templates/default/

If i Upgrade my Cb subscription plugin GPL license new version means, i will edit the source code of cb subscription?

Yes, GPL is not encoded so full source access is available.

and how can i edit that files?

We do not support core edits. We can not help you with this.

1. Can i upgrade my existing CbSubs 1.0.3 membership/license to CbSubs 3.0 GPL.

Yes, you can upgrade your membership on the Membership tab of your profile here at JP.

2. I believe since CbSubs 3.0 GPL is open source and is not encoded, can i do core edits. If i do would you offer technical support for any extra cost.

You can make core edits, but no we will not provide you support for those core edits or if those core edits are in place.

3. Finally i have the web application already developed using CbSubs 1.0.3. Can i migrate my existing web application developed in CbSubs 1.0.3 to CbSubs 3.0 GPL with ease.

Yes, installing over top existing should auto upgrade find, but you should make a backup just to be safe.