No, the Over the Air Programming will only make the changes needed for the phone to accept voice, text and data on the Page Plus network with your account information. All your other data such as contacts, emails, apps, etc. will remain just as they are. You can do the Over the Air Programming multiple times, there is no cost to you or risk to your data.
What you should be aware of is if you do a “factory reset” or “hard reset”. This is what you should do when you sell the phone. This WILL wipe out all your personal information (the “nuke it” option). You can do the Over the Air Programming in this case to put the phone back on your account but you will lose all your personal data and information. Only do the factory or hard reset if you are selling the phone or the phone has an issue that you can’t fix any other way.