Hi Joshua,
> Does anyone know if it is possible or advisable to replace it with Web AppBuilder Developer Edition 2.7?
No, this is not supported. By attempting to modify the underlying files for Web AppBuilder in Portal, you would be altering the internal Portal for ArcGIS software installation and this will void your tech support license agreement.
Please use Web AppBuilder Developer Edition v2.7 (registered externally) with Portal if you want to use the latest app release.
Hope this helps,