Question

Is Enterprise Library 5.0 backwards compatible with Enterprise Library 4.1?
Can I just change the reference and have it work?

Answer 1

The definitive answer is: maybe.

It really depends on the scenario and how you have used the blocks. In the best case scenario you could replace the assembly references, change the configuration and it should work.

From Microsoft Enterprise Library 5.0 Change Log:

Major architectural revisions in
Enterprise Library to use the
dependency injection mechanism. […]
Enterprise Library no longer uses the
static facades and static classes that
were used in previous versions.
However, these facades and classes are
still included in Enterprise Library
for backwards compatibility with
existing application code, and you can
continue to use the previous approach
to creating Enterprise Library
objects.

Of course, things rarely seem to work out that way so, to help resolve any issues and go through the migration approach, you should definitely read:

• Microsoft Enterprise Library 5.0
  Change Log: Breaking Changes

• Microsoft Enterprise Library 5.0
  Migration Guide.