I've been looking for a long time for something (mod or plugin) that will help me replace blocks more easily.
When I say "replace" I mean (for example) that when I want to replace a particular block with another block, instead of breaking the block and then putting the another one I want, I will just click on a specific block that is in front of me and in my hand there will be a different type of block, and the block I click on will change to the type of block that in my hand
(If someone still didn't understand what I was talking about after this privatized explanation, here's a video that could explain it: )
I realized that there were some Minecraft plugins that did the same thing, but everything I found matched only versions 1.8 and below, and I need something like this for Creative Mode in version 1.12.2
I've not seen a mod like what your after, and especially not for older versions that far back. I've only seen mods that randomly switch to another block in your hotbar for you in 1.15.2 from memory, and otherwise that's builders wand like mods but that's not replacing just placing blocks. To me it sounds like some tweak in a mod but I'm not sure if I've seen it somewhere but don't remember the name of it.
Other than that, maybe something like World Edit or Structurize or Schematica might offer something similar, but I'm only guessing I don't know for sure.
I assume one exists with the feature your looking for but I don't know of any/or remember where I might have seen one that approaches this sadly, you'd have to look around a bit on curseforge or other 'trustworthy' sources to see if one exists, or maybe a server plugin instead of a mod.