how to replace sa2 character models (as of 23rd 10th 2019)
get satools: https://mm.reimuhakurei.net/SA%20Tools.7z
open sa2pc/splitmdl.exe
next, depending on which character model you want to replace, you need select one of the following files:
[PathToSA2]/resource/gd_PC/...
	sonic: 			sonicmdl.prs
	super sonic: 	ssonicmdl.prs
	tails mech: 	twalk1mdl.prs
	tails: 			milesmdl.prs
	knuckles: 		knuckmdl.prs
	shadow: 		teriosmdl.prs
	super shadow: 	sshadowmdl.prs
	eggman mech: 	ewalkmdl.prs
	eggman: 		eggmdl.prs
	rouge: 			rougemdl.prs
	amy: 			amymdl.prs
	metal sonic: 	metalsonicmdl.prs
	tikal: 			ticalmdl.prs
	chaos0: 		chaos0mdl.prs
	chao walker: 	cwalkmdl.prs
	dark  walker: 	dwalkmdl.prs
note: chaos 0 cant be replaced without code due to hardcoded texture animations
set the "big endian" checkbox just below the file path
set the output path where the folder with the files should be output, then select split on the bottom right. you should now have a folder with all model files contained in that PRS (all are somehow part of the character model. some Upgrades, some projectiles, some other accessoires. Usually the main character model is the first file, if not, find out which model is which using SAMDL in satools)

as for textures, the texture file for each mdl.PRS should carry the same name as the model file, but instead of the "mdl" after the character name its "tex", with the same file ending (.prs)
you can add or replace textures using textureEditor in satools
a max texture resolution of 1024x is supported (i think). you can replace the textures or add new ones (considering you enter a free number for the global index. my tip is to use numbers in the range 100 - 700, since nothing uses those ids. for a full list of used global indices, check this link https://mm.reimuhakurei.net/misc/sa2pcgbix.txt ), but it is important that the old/replaced textures are kept in the same order, while any new texture must be placed at the end
if you want to save the texture file, SAVE IT AS A .PAK. This is important as it keeps the quality of the textures and doesnt compress them

now, how do we edit existing models?
first, since satools is still having problems with importing and exporting weighted models into other file formats as of the day that this guide is written, you need to use the blender addon BlenderSASupport ( https://github.com/Justin113D/BlenderSASupport ) and install it ( installation guide on the same page, scroll down).
Now, first you need to get the correct model file to edit. All model files are in the output folder, named after the split' prs file. Usually its the first .sa2mdl file in the folder. Import that file into blender using the addon (file -> import -> adv model (.*mdl), might change with time), and, well there you have the armature + all meshes.
Now, you can either export the model and armature into some other format and edit it externally if you are no friend of blender. Simply go to file -> export in the top left and select a format of your choice (I do recommend to either use DAE or FBX, but if you use fbx, make sure to disable leaf bones in the options in the bottom left, as it would add more bones, which we dont want)
About editing the meshes now, there are a few rules to follow:
- Dont touch the bones. Dont rename them, dont move them, just leave them as they are.
- When attaching new models to the armature, make sure that all bones that had weights on any mesh before obtain new weights on the new meshes. IT IS IMPORTANT THAT ALL BONES THAT HAD WEIGHTS BEFORE HAVE WEIGHTS IN THE MODIFIED/NEW MODEL AT ALL
- you dont need to split up the model into smaller pieces like the original model. you can have one big model or many tiny ones, but having less models could raise the converting time later on. It is just important that the bones that had weights before have weights in the new models
- Alternatively, if you modify a file that doesnt create an armature, you need to replace the meshes of the models, but the models need to have the same name as when importing!
once you are done following those rules when editing the models, you can export the model back into blender if you used an external editing program. In blender then, you can edit the materials using the SA material properties panel in the material editor. Each material has its own properties (look up how to select materials in blender if you dont know how). Once you are done with that, you are ready to export the model again. Go to file -> export -> SA formats and select SA2 Model, and export it into the file you originally imported (so, keep the original file name) as the entire folder will be used for the mod

Now you are ready to assemble the mod. First, you need to have mainmemories sa2 mod loader installed ( http://mm.reimuhakurei.net/sa2mods/SA2ModLoader.7z just drag and drop all the files in this zip into the sonic adventure 2 folder). There, in the mods folder, you need to create a folder for your new mod. In that folder, create a .ini file (create a txt document and save it as a .ini). In that ini, you need to write following:
Name=[YourModName]

additionally, you can add:
Description=[YourModDescription]
Version=[VersionNumber (like 1.0)]
Author=[AuthorName]

now, create a folder called "gd_PC" and put your folder containing all sa2mdl files in there. It is important that all files are named the same as when importing, and that the folder has the same name as the PRS file that was Split before
If you replaced or added textures, create a folder called "PRS" in the gd_PC folder and put the .PAK in there. That should do the job

Now, if everything was done correctly, you should be able to activate the mod in the mod loader and boot the game. If the game crashes on the title screen, try a few times again, thats just how sa2 behaves :v Only if the game crashes while loading a stage with the character, that is when you know you did something wrong

Happy modding! ~Justin113D