DD4T2.0's CD side is backwards compatible with DD4T 1.x
This is designed specifically for upgrade scenarios where customers may have thousands of DCPs and Pages published using DD4T 1.x's XML structure and can't easily republish the entire site as part of the upgrade.
What I mean is, the new framework is still able to deserialize XMLs published by the old framework.
So, you would need to upgrade:
- on the CM, for every template switch out the old DD4T TBBs with new ones - if you want to do the CM.
- on the CD, replace old dd4t DLLs with new ones.
- look at how the web.config settings map from old to new, particularly around cache configurations. DD4T2.0 has a newly added mechanism, but should still have the old one.
- recompile your app against the new framework. If you run into errors due to API difference, post them here as separate questions. Otherwise, do some regression testing on your app and you should be good to go.