I orginally expanded the description of the four techniques, but felt that it didn't belong in this blog article, but for those who want more, here it is.

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Retirement is obvious, but its unusual that you can retire the whole of an application, however, some parts of an application might be migrated to new infrastructure components, and these days, more likely remote services, is this called re-factoring these days?. Printing is an obvious candidate, and the customer file a less obvious one, although most e-commerce applications should be using someone else's code to manage the customer file. (LDAP, Active Directory or even a web service.)

Emulation is more subtle, because it varies from black box emulation environments, which might be necessary to migrate language environments such as PICK, MUMPS, or RPG or even BASIC. A black box interpreter is provided and the code, business logic and data migrated, hopefully by copying. Emulation is also the name I apply to the migration of more modern  modern interpretive environments such as a DBMS or even a COTS package. One relies on the porting guarantees of the vendor/author to ensure that the environment works on the new platform and then moves the code, business logic and data. This technique supports the porting of applications between operating systems.

The latter two techniques can be used to migrate the business logic at higher levels of abstraction, between, say two database implementations or 4GLs. For instance, there was a Wang PACE RE tool, that generated Powerbuilder/SQL code, it even continued to look like PACE screens. I suppose these two techniques are on a continuum and it depends on how many tools you use to conduct the work as to whether you consider it code engineering or reverse engineering. The latter usually involves a tool. Most of the database vendors have a tool for reverse engineering the database. The problem comes when one considers the proprietary usually procedural objects such as stored procedures, batch sql, or triggers, which is where people are needed.
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You can also still buy the book, if you want :)