Does this book cover migrating applications from Struts 1.x to Struts 2.x?
No, it doesn't.
Is it feasible to migrate or are the paradigms very different?
Struts 2 is pretty different, possibly making a straight migration time-consuming. On the conceptual front the differences are relatively minor, but there's a fair amount of busy-work involved. There's no reason the same web app can't use both Struts 1 *and* Struts 2, making an incremental migration relatively straight-forward.
I've migrated two applications; the biggest differences were the tag libraries (largely mechanical translation), the action classes themselves (Struts 2 is much cleaner; the ease of conversion of the Java code depends largely on how clean the Struts 1 code is), and some validation differences.
Does the book draw comparisions with Struts 1.x so that developers moving from Struts 1.x to 2.x can benefit from their existing knowledge?
Only minimally. To be honest, most Struts 1 knowledge isn't particularly helpful, and the S1 knowledge that *is* is general web application knowledge. There are few similarities between the two frameworks.
The Struts 2 documentation wiki contains a couple pages which might be of interest to those converting an existing application: