Prestashop migration 1.2.5 - 1.4.6.2

Hi, Currently running Prestashop 1.2.5.0 with a large number of products and images stored. Server is very basic andmay erquire complete down and upload, so please only apply if ou have a good broadband connection. We need a sample conversion to a copy and then a time estimate how long the shop needs to be down for the actual migration. All data needs to be migrated.