SEO penalty Avoidance question when changing Blog URL and Backposting

I am changnig my blog's URL and host. I had about 180 posts and wish to
transfer about 30 to the new site which I can do by Wordpress. I am copy and
pasting (yes I know real crazy but I don't know php) I want to backdate
these posts to the date I originally published them on my old blog.

Question:
1. Since my new domain's start date is in Jan 08 and I am backposting them
back a few months prior to the start of the new blog, will I incur a penalty
from the Search Engines? It is compounded also in that you cannot just
"save" the posts with a backdated timestamp. Saving posts makes them merely
drafts and not visable on the Blog. If I "publish" them they get sent out
via RSS with a date that is older than my new domain in the timestamp.
2. There is then duplicate content from my new RSS stream with the RSS
stream of my old blog which I assume is somewhere in the google cache!

Perhaps I should just take the ideas and put them in new posts and accept I
have to start from the beginning since I am changing the URL? Through I hate
that.