<p>Suppose I use the [RemoteClass] tag to endow a custom Flex class with serialization intelligence. </p>

<p>What happens when I need to change my object (add a new field, remove a field, rename a field, etc)?</p>

<p>Is there a design pattern for handling this in an elegant way?</p>

Adding or removing generally works.
You'll get runtime warnings in your trace about properties either being missing or not found, but any data that is transferred and has a place to go will still get there. You need to keep this in mind while developing as not all your fields might have valid data.
Changing types, doesn't work so well and will often result in run time exceptions.
I like to use explicit data transfer objects and not to persist my actual data model that's used throughout the app.  Then your translation from DTO->Model can take version differences into account.