Giovanni,

> Is there a way to change at runtime the class of an instance?

The alternative State behavioral pattern that Ned suggested is in  "The
Design Patterns Smalltalk Companion" by Alpert, Brown and Woolf. If you use
Smalltalk, this is a useful book.

If you want a peak at the STATE Behavioral Patterns from the book, you might
sneak over to:

http://members.aol.com/kgb1001001/Companion/state.pdf

Where there happens to be such a chapter from the book by one of the
authors. However, you should buy the book, it's useful to clutter up your
bookshelf with.

Jim

PS: Several different chapters are on line:

http://members.aol.com/kgb1001001/Companion/Companion.htm