Factory or something else?

I'm trying to think of a solution but not sure which way to go. Here's the problem:

Its going to be a library for parsing multiple XML files. So I know what classes to make and how they are going to be nested. A year or two from now, the software that emits the XML file may change the syntax of the XML file. And at that point of time, my solution has to handle two different set of XML files based on the version written into the main XML file. What I would like my solution to do is,
a.) Check the version of the software emitting XML files and call the appropriate Parse routine, say Version1.Parse() or Version2.Parse(), that then calls the parse routine of different files, like ParseFileA() or ParseFileB().

b.) There is a possibility that of the 20 XML files, syntax for only 5 of them has changed (ver 1.1) and rest remain the same. In that case, I would like to call the different file parsing routines. I guess, I should be able to call the appropriate method based on the version read in the previous step through a conditional branching (just wanted to let know).

From the look of it, Abstract factory seems to be the pattern to go ahead with. But I also feel that I should be defining the parse methods through an Interface rather than through an abstract class.

Am I right? What approach would you have taken?
Thanks.