Hello Autme;

An integrated WinCC Flexible project cannot be directly migrated to tIA Portal. You will have to take it out of the integrated project (copy to....) and then mgrate it to the specific level of WinCC TIA Portal required:Basic, Comfort, Advanced or Professional. 

Here is a document discussing migration of intergted WinCC Flex prjects, and as an attachment is a FAQ giving an example of migrating an integrated MP 277 to  TP 1200 Comfort panel, in TIA Portal v15.

https://support.industry.siemens.com/cs/us/en/view/77430539

Hope this helps,

Daniel Chartier