XML Parser Message: Element series is not valid for the content model

Hello,

I work with FrameMaker 8 and DITA.

I change the element prodinfo in the topic.edd from:

General rule: (prodname), (vrmlist), (brand | series | platform | prognum | featnum | component)*

to:

General rule: (brand | series | platform | component)*

When I import the element definition to the template everything is okay.
When I insert the elements metadata, prodinfo, brand, series, platform and component into a topic I get the XML Parser Message that the element brand is not valid for the content model (prodname,vrmlist, ((brand|series|platform|prognum|featnum|component))*).
When I delete the element brand in the topic I get the XML Parser Message that the element series is not valid for the content model (prodname,vrmlist, ((brand|series|platform|prognum|featnum|component))*).

I change the element prodinfo in the topic.edd to:

General rule: (brand)?, (series)?, (platform)?, (component)?

...and get the same Parser Message.

I do not understand that. Is not it allowed to change the EDD this way without changing the DTD?

With kind regards

Nina