301 redirect of old site to new

We currently have a website that is running on a very old version of oscommerce. This older version website is ranked very high in the search engines which we do not want to lose in the conversion to the new site. The new site runs on Creloaded has different page names, but has not been optimized since we were always intending for this new site which is live and running to be the replacement of the older version. The problem, both sites are live, taking orders and on different URL's and different servers We can not have down time on either, or at least downtime can only be with the hours of 11pm est and 5am est We need to replace the old site with the new site design and make sure that the re-direct does not eliminate the ranking. If possible we would like to move the old site to the new server, and have both sites and any future sites run from the same database (not sure again if that is possible) Since this is a bit of a sticky situation we will not be posting an exact amount for the bidding, but based on sugguestions and recommedations from bidders, what would need to be done we will add to the budget. Our main concern is not to lose our ranking, nor to go offline for a larger amount of time. Please do not contact us via phone or ask us to call. All questions will be answered via email only.