Once you change your phone number, you won't be able to view online any unbilled call or messaging records for your old number. You have to wait until the bill is ready.
Once the bill is generated online, you can go sign into My Verizon and click on My Bill, then click on Bill Details link below and click on the Data Details. You will be able to to use the drop down box to select your old mobile number so you get the text records for that number.
VZW unfortunately can not provide you with unbilled billing records unless you get a court subpoena due to privacy regulations.