I went through pretty much the same thing - test DID etc.. and then ported my number.
The wiki explains it clearly: »wiki.voip.ms/article/Porting_a_Number
Also, when you initiate the process, the instructions are pretty detailed. If you're not sure of something, just open a ticket with voip.ms - I wasn't sure about something specific to my situation and asked them, got a reply relatively quickly.
Once the port is completed and have confirmed that everything works fine, you can go to "DID numbers/DID billing" and delete your test DID from there. Just make sure you delete the test, not the "real" one.
One important thing to keep in mind is to not cancel your service with your current phone company until the port has been completed (I've heard of some people who did that without problems, but it's stated in the instructions and I did confirm that with voip.ms before initiating the porting procedure).
I don't have an answer to your question about the balance, though you should make sure you have enough money in your account to cover the $25 porting fee.