Two issues really exist:

1. Backwards Compatibility
2. Analytics Redux


**Backwards Compatibility**

There is a series of upgrade scripts, procedures, and steps in order to upgrade ECM 2.2 to EXM 3.0. *I will edit with link to the Upgrade Doc*.

However, as of EXM 3.1, backwards compatibility to utilize ECM links, clicks, opens, and bounces was removed. Along with it, reliance on any of the old `AutomationStates` table.

The real issue is that the links created out of ECM contain `ec_as` and `ec_camp`. These correspond to Automation State and Campaign Id. 

In EXM, those were removed and now `ec_message_id` and `ec_contact_id`.

Well, if you have access to the old Analytics database from 7.x, you *can* derive a Membership Profile username.  But, EXM utilizes xDB contacts.  The upgrade scripts creates user accounts in xDB, but the identifier is **domain\username** and OOTB EXM wants to use **email address** as primary identifier. So, without close consideration paid attention to, it's possible to have two Experience Profile accounts for the same person, but different identifiers which causes a host of issues.

So, with EXM 3.3, there's no way to get an xDB Contact from the `ec_as` query param without heavy reverse engineering.

Similarly, you can use the `AutomationStates` table and `Automation` table to derive the email message ID.

Knowing that, you can create pipeline processors for `<emailOpen>`, `<handleBounce>`, and `<redirectUrl>` to massage the params and find the contact I'd and message id. But again, you have to build the backwards compatibility.

Lastly, ECM OOTB utilizes the `modules_shell` site and a ASPX to handle RegiaterEmailOpen. In EXM, this page doesn't exist, and instead moved to a handler (ASHX) that lives in `/sitecore`. So we had to rebuild the old page functionality but refactor it for EXM and xDB.


**Analytics Redux**

EDIT: Anthony describes the story around analytics way better than I. Refer to his response.

**SUMMARY**

We were contracted by a client doing just that to rebuild the backwards compatibility from ECM to EXM 3.3 It was not an easy lift, and resulted in a number of Support tickets. EXM 3.4 will be coming out sometime in the near future which will support Sitecore 8.2, and I've been told that it's a pretty light weight upgrade from EXM 3.3. 

Therefore, if coming from ECM 2.2, and going to EXM 3.3, I advocate working with a Sitecore partner to really hone in on what matters most and figuring out a path.

That's about as much advice as I can give.