I program numerical models to solve Partial Differential Equations in Fortran, in serial and parallel (with MPI). Only Fortran, I do not know/need other languages. I see I now need to migrate to Intel OneAPI, before I had Parallel Studio XE 2019. Any advantage/new feature after migrating to OneAPI that a Fortran average user will enjoy? I never used GPUs, will OneAPI make the transition easier if in the future I wanna learn how to parallelize the code and run it on GPU?
There is no necessity for a transition to speak about. You will get the same ifort you had before, just an updated version. And an option to try the new (LLVM-based) ifx, but it is just an option, I did not use it yet either.
The ifort compiler is the same compiler you are used to, just updated to (as of now) version 2021.2 with various small improvements and bug fixes as always with their new version.
If you do want to try new ifx, it indeed comes with new GPU features. Only the ifx compiler supports GPU offload. See Get Started with OpenMP* Offload to GPU for the Intel® oneAPI DPC/C++ Compiler and Intel® Fortran Compiler (Beta)