It should be, as long as you have a source with the toon shader in it compiled on your drive (because that’s the only requirement; you need to have an UE4 source code compiled somewhere). The only thing you need to do is switch the current engine version of your project with the new one. To do that, just right click the uproject file and switch the engine version to the one with the toon code in it (can be done for both blueprint based as well as C++ based projects). You may need to reboot the launcher (your project will then show up with “Other” instead of the version of the engine you were using previously), but after that just launch your project and have fun with it.
I tested it on both my most recent C++ based project (moved from version 4.9 to 4.10 with toon integrated) as well as an older blueprint based project (from 4.7.6 to 4.10 with toon integrated), and I got the toon shader working in both instances.