Comma after address

Both versions are imperative clauses, and both have directive force. The difference is: your first version uses a 3rd person subject, while the second version uses a vocative.

In both versions, there is the directive "Rejoice!"

Both versions basically have the same meaning. In a roomful of chocolate lovers, you can give the directive "Rejoice!" or the directive "Chocolate lovers rejoice!" or the directive "Chocolate lovers, rejoice!", or the directive "Everybody rejoice!", or the directive "Everybody, rejoice!" or the directive "Rejoice, everybody!"

For more info, there's the 2002 reference grammar by Huddleston and Pullum et al., The Cambridge Grammar of the English Language, Chapter 10, pages 925-8 (9.2.1 - 9.2.2) which includes the section "Subject vs vocative in imperatives".

Yes you are correct. The comma is essential in the second example. It is a bit like 'Let's eat grandma!' versus 'Let's eat, grandma!'

The first means that chocolate lovers are rejoicing, whereas the second means that chocolate lovers are to rejoice.

1.) A statement 2.) A command

Therefore, you cannot use them interchangeably.