Last week I upgraded from 2.2.5 to 3.0.5 Pro utilizing the Store Import Tool, then spent a few days tweaking the new store & am now ready to go live on the new 3.0.5 store.
Of course since the original import was performed, there are now several additional orders & new users which I will also need to migrate to the 3.0.5 site just before the final flipping of the switch.
So, question is what method did you guys running live sites use, to do the final migration of customers/orders when actually ready to go live?
Is it possible to use the Store Import Tool a 2nd time to migrate this final data, or is it an all or nothing scenario as it appears which will also override my new site changes in place? 
Thanks!
[quote]Is it possible to use the Store Import Tool a 2nd time to migrate this final data, or is it an all or nothing scenario as it appears which will also override my new site changes in place?[/quote]
Well, yes, you can do this, however, I do not suggest anyone else attempt doing the final migration using the Store Import Tool as it will create havoc and several hours of additional work getting everything put back into place …
Although on a positive note, the upgraded site is running like a finely tuned machine & ready to accept orders at the crack of dawn, too bad I won’t be awake to see them roll in! 
I closed the store during the migration to avoid this problem. Closing for maintenance was a one day event that saved a lot of the headache you are going to experience now.
You could select the new orders and customers via SQL query and export the data via phpMyAdmin, in CSV files. Then import them into the new database for v3. I did my entire migration like this - before the store import tool was created.
But, you don't want an order conflict with NEW orders in v3 either - I would still close the store to prevent this during the final order import.
If you do it manually, just make certain you find all the data in each table for the orders and customers.
Thanks for the reply Don,
I managed to get the site upgraded and back to live before late morning rush, although no doubt I chose the hard way to go about it this time around! 