I've been developing and submitting iOS Apps for 9 years now, so I have a lot of experience with Xcode, and making any change required when migrating to a new version of iOS.
However, it won't be clear exactly how long this migration will take, because at this stage I don't know which APIs or 3rd Party libraries you are using in your project.
The best case scenario is that your project will merely require compiling in Xcode and then submitting to the App Store, the worst case scenario is that your project uses 3rd party libraries which are tied to a version of iOS that Xcode no longer supports, or an chip architecture which is now longer supported ( e.g. 32 bit ARM6)
Thanks,
Ben