The only requirements for IPB 4.x is to have mysql 5.3 or higher and php 5.4 through 7.
7.1 should work, I do not have this confirmation, but if you have the power to change the version from 7.1 to a smaller one, then do so.
If you have these versions on the server you are enabled to send everything without errors. So by FTP you have to send everything, but if your server has a connection time limiter, you will have to send the files bit by bit.
I'm not fully understanding what you're doing so far. But again I say that you need to delete or replace all uploaded files and re-upload, one folder at a time so the process will not be interrupted.