I have one app that is already live on AppStore and it is developed using ionic. Now we decided to re-develop the same app using another platform like native or flutter. Is it possible to upload the build to the same app created in Appstore connect?
Yes, it is possible and quite common.
A new version of your application can replace the old one regardless of differences in code base, visual design or logic. Technically speaking you could upload a new version which is a completely different application from the previous one and users would see it as an update.
This specific example of yours happens a lot. Application is at one time developed with some cross platform tools and then later with native tools or vice versa. Or sometimes even the same platform is used but a complete rewrite is being made.
One of challenges you may face though is how to preserve local user data (if you have any). If your current application stores some user-generated data into some database then the new application needs to be able to hook into the same database. Alternative is to have an intermediate update using Ionic which prepares data for migration to the new version (drop everything into some JSON on local storage for instance).