So I wanted to start from scratch and make a new magento folder (with magento 1.9.1 ) for my existing site (which runs on 1.7.0.2) and eventually point it to this new folder once it is done. Basically I do not want to have downtime while I am working on the new one.
Question: How hard (or easy) is it to import order history, customers and newsletter subscribers to the new database?
In my opinion: just re-use the database. Make a copy of the database, and point the new install to it. It should have all your old data with the new functionality in it.
If you want to exclude product and/or other data, it gets more difficult. But usually, it is possible with partial SQL dumps and imports, just be careful to always have backups at hand. In general: if you only want customers, newsletter subscribers and orders, you should export database tables starting with customer, newsletter and sales and import them into your new database.
from my point of view order import will complex process.But you can import customer and newletters subscriber.
For customer export to goes to Your old (1.7) magento instance admin>System>Import/Export>Export and select Entity Type =customer export all customer.
Import the customer to 1.9.0 magento from admin>System>Import/Export>Import and select Entity Type =customer import all customer.
Note: It did create same customer id like old magento 1.7
