I don't know much about Textra but any message that comes thru your carrier goes thru your native SMS app on your device. If you download a third party app, something from the play store, it still goes thru this native app. They sync so if you delete something it gets deleted in both places but you can install and uninstall SMS apps all day and your messages will always be in that native app, until you delete one or some or whatever. So the long and short is no you won't lose any messages that you don't delete just trying an SMS even if you uninstall it later. The only way you would have messages that wouldn't be held would be if you installed a messaging service or an app designed to erase messages automatically.