I am in the process of investigating a domain name change as I do not own my dot com, I do own the dot net so for my own security are considering a switch.
I have a dedicated server who has told me that he can setup redirects and a "DNS zone" so people can get either URL for a while.
What would be the effects of this to the readers and Google? Will they penalize the site for duplicate content? Will they switch over to using the .net? Any ideas on a timescale to keep the redirects up? I don't want to switch instantly for obvious reasons, Google hates new domains and we'll probably loose the lot.
You'll want to do a 301 redirect (permanent redirect). This points your old domain name to the new domain name while still preserving all the links you have from other external websites. By doing a quick search for 301 redirects on Google you will get plenty of info on how to do it correctly. No need to do a temporary switch over so that both work for a short time. Just switch it all at once and make sure all the proper redirects are in place when you pull the switch.