When I finish my trial and sign on to a paid account, will all of the changes and customizations I made to my trial account be lost?
All of the customizations and changes you already made in your trial stay with you when you move to a paid account. This applies to everything from custom triggers to the Talk line that is associated with your account.
If you don't want the customizations you made on the trial to transfer over, create a new trial and turn it into a paid account right away. See how to do that in this article: How do I upgrade my trial to a paid account? To keep the same subdomain in your new trial, follow the directions in this article: Can I get my old subdomain back?
If you want to keep your customizations but start fresh with none of your test tickets, you can follow the directions in this article to bulk delete the tickets in your trial account: Bulk deleting tickets.