Nickitkat

UPDATE:

I figured out how to make it work. So, I am leaving this here in hopes that it will help you instead. 

There are 2 methods I have discovered. But you must first:
Enable creative mode Give yourself a Magic Wand Press "~" Select 'Select Tool' on the left toolbar While using magic wand. Left click to set first selection point, right click to set final selection point. The end result is a highlighted area showing your selection 'zone'. This zone should encompass all the blocks/structure/buildings that you wish to swap blocks. (Note that when having the ~ menu open you can't look around, so make your first selection then close the menu, fly to your second point and open the menu and make your second selection) Use the U+ U- D+ D- buttons to set your selection's Z axis. Also, you can adjust your selection zone size using the N E S W +- buttons. These buttons are located at the top of your screen Change the 'Constrain' drop down menu to 'On' - Important!  Done. Now moving on to the 2 methods. Method 1: Swapping Blocks using '/we repmat' Command
With your selection zone set, make sure: Hotbar 0 is the block you want to be swapped. The block that you want it to swap to can be at any hotbar but you must be selecting it.  Type /we repmat All the blocks in hotbar 0 in your zone will swap to the block in your active selection hotbar. For example if you wish to swap granite cobblestone to andesite cobblestone. Granite cobblestone should be in hotbar 0, andesite cobblestone can be in any hotbar as long as it is selected. Type the command and it should swap. This will swap chiseled blocks as well. If you have a chiseled pine block with some granite voxels on it, the granite will change to andesite when you run this command.  If this method doesn't work for some reason, move to Method 2.   
Method 2: Swapping Blocks using Paintbrush in WorldEdit (Pressing '~')
If Method 1 doesn't work, then press '~' and select 'Paintbrush' button on the left Set your desired paintbrush size by adjusting the values of the SizeX,Y,Z Now, with the block you wish to swap to in your active hotbar, right click on the block you wish to swap  This will place the block, but it will immediately disappear and the block you wish to swap will all swap to what you want, but only the same block type within the paintbrush's selection area will be affected. If it doesn't work, it is most likely because something is preventing the block from being placed down. It has to be able to be placed down adjacent to the targeted block first before being able to affect all other blocks. Note
So far, both method doesn't work for doors, trapdoors, fences, slabs and maybe other blocks which I haven't tested. These will have to be manually replaced.  Sometimes, chiseled blocks doesn't swap. In this case, you might have to download QP's chisel mod to ease the process of transferring chisel patterns to a different block assuming your chisel work doesn't involve other block types and is purely just form.  P.S - Thank you @Professor Dragon for replying to my query. You have been very helpful 🙏.

I hope this helps you fellow Vintarian.