Total upgrade process time varies based on server (speed, compatibility, size of database, amount of content, etc). I can usually limit actual downtime to less than 10 minutes. Most of the work is back end preparation. There can always be extenuating circumstances that may cause longer downtime, but I've never had one down longer than a couple hours. Alternatively, a safer option would be for me to clone your site to a subdomain and upgrade there. This way you can work on the new site without taking down your live site. When ready to move live, I can then reconcile any new customers and orders and move your new site live. Please contact me for a quote on this additional work.

What is included in this service
- Migrating Products
- Migrating Category
- Migrating Customers
- Migrating Orders
- Migrating Customer Groups
- Migrating Attributes and Attribute Groups
- Migrating Options

Things I cannot upgrade:

- 3rd Party Modules (you need to check if the developer upgrade to 2 - 3.x.x the module)
- Template that are not compatible with OpenCart 2 - 3.x.x

You can also reach me out at asktofaizan@gmail.com for discussing it before buying my service. cost of migration depends on size of database, and number of extensions and the theme design needs for the upgrade.