Is it possible to replace vanilla textures?

Yes it is possible! (And sorry for my bad English!!)

You have to go to your MCreator Workspace (Workspace->Open Workspace) than go to src -> main -> rescurces -> assets.

Here you see the folder nemed "(Name of your Mod)"  and than you have to make a folder named "minecraft" (in this folder you put from now on all your textures or thinks that you want to change from Minecraft like tha Armor). Here you create a Folder named "textures" and past your Texture of your Item/Armor(!!only how it looks in your Hande!!)/Food... (This is the Texture how it looks in your and ingame).

!Warning! - if you make a texture for Armor you also have to create a Armor Model. You also have to creato a new Folder in the "textures" Folder. The Folder should named "models" in the "models" folder you have to create another Folder named "armor" and than inside this Folder you have to past the Armor Model Texture you have made (the armor Model Texture is the Texture how it looks if you put the Armor on. I will make examples for the "in Hand texture" and the Armor Model Texture!)

The Armor Model Texture has always two Layers!

By the way, here are the names you have to name your Armor(I don't know wich you want to change so I will give all examples):

-Chainmail:

   - chainmail_layer_1.png(and: chainmail_layer_2.png)

-diamond:

   - diamond_layer_1.png(and: diamond_layer_2.png)

-gold:

   - gold_layer_1.png(and: gold_layer_2.png)

-iron:

   - iron_layer_1.png(and: iron_layer_2.png)

-leather:

   - leather_layer_1.png(and: leather_layer_2.png)

-netherite:

   - netherite_layer_1.png(and: netherite_layer_2.png)

So here are the examples:

(Idk why but I can't insert Pictures pleas tell me how it works)

Hpoe I could help, if no, say me hoe to upload Pictures and I will help you :)

Really? I. An help you if you still need help.I know it is late, but I know how to insert Pictures yet, so I can make pictures for you if you still need help!