The P3D model is very different from the old FS9 model, so those textures will not match even if they look similar.
I think that the simplest way to convert your old repaints would be to resize them to 2048 px resolution, then import them as layers in the repaint kit for version 5.x and work from there. The template layer will give you some guidance.
Good luck, and feel free to share the results here!
Win 10 Pro | P3D v5, v4 | AMD Ryzen 3 3.5 GHz | 32 GB RAM | NVIDIA GT1030 | TM HOTAS Warthog | TM HOTAS Cougar