magento store version upgrade, magento expert needed.

we have magento based store running on our website. we want to upgrade to version 1.70(latest magento version) current version files has so many changes and custom modifications. we will to have fresh installation of magento and wish to keep existing product and customer database. means keeping current database of store you need to install fresh version. (no upgrade to current files, just install new and use the current versions sql database.) note-store is running live and has few hundred visits per day so you need to manage all work in seperate folder and once done then move the work to main site so that main website may not be offline and shows no error during all this process.