There should be no need for downtime or mainteinance. It depends on the site, but it is common that we only need to update the files from the new version (maybe, just a little schema update)
Relevant Skills and Experience
I've worked with multiple Drupal sites. I'm very experienced in creating, migrating, updating or changing sites using Drupal
Proposed Milestones
$100 USD - Project Completion
Hi,
It is usually not a very complex task to update a Drupal site.
If you feel more comfortable with it, I'm offering you to copy the site, so we can update it in a controlled environment where we could check everything is working fine, and then upgrading the site.
Let me know if you are interested.
Regards