If you are using a release before 0803, adding the new domain should be as adding the new domain to the relevant configuration settings and changing the accounts as Yousef has mentioned. Have a look at the following KB if you have access:
http://kb.k2workflow.com/articles/kb000182.aspx

If you are using 0803, just rerunning the Config Manager should reset the domain settings and apply the new domain configuration. Remember to assign permissions to users in the new domain as well.

For running instances it can be a bit tricky and I would recommend that you complete all running processes prior to the migration if possible. The reason for this is that the Destination Users are kept in the database for tracking, resolving, etc. and part of the user name includes of course, the domain.

If this is not possible, you can manually redirect the Worklist items after migration to users in the new domain. The final option is to get the K2 guys involved, as the only alternative is to update some of the tables and making any databases changes yourself will of course void support.

A final thought, things like Reports will still reflect users from the old domain, so it may confuse some of the business users initially.

The KB article mentioned in this post URL is http://help.k2.com/kb000182