Can I have lazy="false" in my collection mapping, but in some cases, during runtime, set it to lazy="true"?

Or the other way around?

Depending on what your reason for doing so is, you can instantiate the items in a lazy collection simply by issuing Collection<t>.Count while the parent object is attached to a session.

Use SetFetchMode() in the ICriteria interface or "join fetch" in HQL. Have a read of the docs about fetching strategies here:
http://www.hibernate.org/hib_docs/nhibe ... e-fetching