There are several ways to run a portion of your changelog.
If you break up your changelog files using versions (changelog-1.0.xml, changelog-1.1.xml, changelog-2.0.xml) and then have a master changelog.xml that <include>'s them, then the easiest solution for you may be to just run updateSql passing in the changelog version you want. If you want to generate sql for the entire database, run "liquibase --changelogFile=master.changelog.xml updateSql". If you want to generate sql for just version 2.1, run "liquibase --changeLogFile=changelog-2.1.xml updateSql"
@Jens answer works well and is what I often suggest. It has the advantage compared to the above option of catching new changeSets that were introduced into old changelog versions through a merged in patch release, for example.
Beyond that, you could use context or preconditions to dynamically control what is ran. Depending on your setup there may be ways to get those to work well for you.
Finally, there is always the extension system where you can write custom logic around how changelogs are parsed and executed to pull out older version changeSets based on file name or some other mechanism that works for you.