Hi there, im Alex, Prestashop expert. I recently finished a project on freelancer where i migrated 3 sites, 1.4 1.5 and 1.6 versions to one multistore with one back office. I can show you old sites and the new one so you could see that all data were migrated.
So basically i migrated this data:
Orders
Customers
Products
And all went well. I can do this for you too.
Do you already have multistore up and running?
Thanks and lets have a chat to proceed.
Alex