I was looking for that feature in ASTRO, too. Would be great if anyone knows.

But this thread belongs into the Application section.

 

I found this:
http://www.targovetul.info/2010/12/how-to-change-file-association-on.html

I think that is the only way

I couldn't copy paste here due to copyright claims

 

Go into Settings-} Applications-}Manage applications

Long tap on Application that has control.

Press clear defaults.

Next time you call the association you will get the list of possible apps.

That box will pop up until you select a new default