Actually, the version 2 is not compatible with the old configuration. Please find the docs here.
Please find very condensed summary of changes in binding version 2 in the section Changes from Modbus 1.x binding. Quoting the introduction:
The older Modbus binding is quite different to the new binding. Major difference is the thing structure introduced in new openHAB2 binding which allows configuration uisng the PaperUI.
Unfortunately there is no conversion tool to convert old configurations to new thing structure.
Due to the introduction of things, the configuration was bound to be backwards incompatible. This offered opportunity to simplify some aspects of configuration.
As you are probably are familiar with basic concepts of Modbus, I recommend reading the docs through and get an overview and understanding of the logic with the new binding. The examples are likely useful to get started fast.
I need to check if a modbus slave is online or not.
There are new channels telling information of last successful read/write as well as last erroring read/write. Using these you can decide when the slave is down. It might be also to react to thing status changes in rules, but haven’t tried it personally.