Thanks for the post, I would recommend to check with the App Review team as the final decision always rests with Apple's App Review team, your proposed approach intersects with several strict App Store Review Guidelines I can see.
Are you just making those changes into an app or creating a new copy of the app?
Like the rule to prohibits cluttering the App Store with multiple versions of the same app. If the original open-source app is already on the App Store, or if other developers have used the same open-source code to create their own branded versions, Apple will likely flag your app as Spam.
I don’t think you are using a template as apps created from a commercialized template or app generation service will be rejected unless they are submitted directly by the provider of the app’s content.
The expectation is developers to submit original ideas. If the open-source app is a clone of a popular app, or if your reskin makes it look like you are trying to impersonate another app, it will be rejected under the copycat guideline.
We recommend that you sign up for a session with App Review during the weekly Meet with Apple event. Sign in with your Developer ID and select "Request a one-on-one App Review consultation". A member of the App Review team will help you with your questions regarding the review process and the App Review Guidelines.
Or contact them https://developer.apple.com/contact/#!/topic/select
Hope this helps.
Albert WWDR