The reality is, it might be quite simple to convert an SD 3D game into an HD one. When you create a model in a 3D graphics engine (as I'm learning with 3DS Max) you're not creating ever single polygon from scratch, you're usually creating more complex shapes, and then defining settings on how smooth and complex the processing on the model will be. So you create a sphere, which could have an infinite amount of polygons if you wanted it to, and then you set how complex it is based on what you wish your processing load to be. The engine behind Final Fantasy X likely works in a similar way, it's sent a bunch of mathematical models, and then set at a certain level of complexity that the PS2 can handle.
I'm not saying it's as easy as going in and changing a couple of parameters and calling it good... we're probably talking about changing parameters on every 3D model, for starters. But it's FAR easier than creating a bunch of models from scratch. Texture resolution is probably even simpler. Most 2D graphic designers create graphics in far higher definition than the final outcome will be, because they can, and just in case. Especially since HD was right on the horizon during FFX creation, you can be sure the texture artists were creating fairly high resolution raster, or even simpler, lots of vector graphics, in which resolution means nothing anyway. THAT would be just as simple as re-converting all of the texture files from their original Photoshop, Illustrator, or whatever created them files, and saving over the old ones. I mean, in this case, we really are talking dead stupid easy processes. Similarly, the games 2D graphics would already be higher resolution, probably including the pre-rendered 3D models (which quite a few interiors were).
If you plan ahead, you can create a game for any resolution and processing level, and in fairly short order, switch it over at a fraction of the time/cost it would take to create a new game. You can be sure that the type of engine used to create FFX is only a small difference between the engines of today. FF7 on the other hand, would be a completely different matter, using totally different kind of graphics, with probably no thought in planning for high resolution (since that wasn't even on the horizon). The models weren't simplified down from more complex mathematical functions, you can tell they were build from the ground up to be simple. It really wouldn't be any easier to convert FF7 than it would be FF6, in each case, you'd basically have to re-create the game.