I have been working with magento for almost 7 years now, with PHP for about 15 years and with SQL for more than 20 years. Upgrades from magento pre-1.8-versions are a bit more complicated to do and require some more work, because there are some incompatibilities between v1.7 and v1.8.
Thatswhy this upgrade has to be done in two steps: first upgrade to v1.7, and then upgrade to newest 1.9.1, and also it will be done in a seperate staging area of your server with a seperate db, so that your shop has no downtime at all. All Extensions have to bechecked for compatibility, including the two, which you say are crucial to keep. And if there were incompatibilities, replacements must be discussed. Furthermore all customizations done on your site must be checked for compatibility, and a workaround must be found in case.
Only when everything works fine in the staging area, the results will be ported to your live-server. This will be done within a day, or a little longer, if replacements have to be discussed.