I'm trying to replace a bunch of oak trap doors with spruce trap doors, the issue I'm having is that they're all open trap doors and there doesn't appear to be any way to set this block state using the schematicRebuildReplaceAll keybind. Am I missing something?

Right now I'm really wishing there was a key I could press while pointing at a mismatch in my placement that would replace all blocks matching the original block state in the schematic, with the block state at the block I'm pointing at, if that makes sense.

Right now I'm really wishing there was a key I could press while pointing at a mismatch in my placement that would replace all blocks matching the original block state in the schematic, with the block state at the block I'm pointing at

That sounds like a really nice way of adding acceptable replacements or doing rebuild actions, I've noted it down on my TODO list.

Which MC version are you on? I think I added the newer Rebuild mode feature from the 1.12 code base also to some of the 1.15.2 versions, where you can use Alt + middle click to "store" the block state you are looking at in the Rebuild mode (same way it works for the Fill and Replace modes), and then you can use that stored state for the rebuild actions by using an empty main hand, instead of holding the block to use like you would normally.

I don't remember if the version on CurseForge also has that feature, but I think it might. If not, then try the version from here: https://masa.dy.fi/mcmods/client_mods/?mcver=1.15.2

Off-topic: All of these temporarily ported features are getting pretty messy because I can't remember which builds have which features, and they are not part of the main branches for each MC version because they are just cherry-picked and temporarily ported from 1.12, before I'll do the next actual full merge from 1.12 forward... but before that there is still also a whole bunch of stuff still on my TODO list that I want to add, so this situation has been going on for a long time already...

I'm currently on the 1.14.4 Fabric version from CurseForge, 0.0.0-dev.20191222.014040. The alt click thing from the 1.15 builds does look useful though.

If you just want to change the blocks in a schematic for some project for now, you could do that in the 1.15.2 version, as the schematic format hasn't changed since 1.13.2, and I think there shouldn't be any or at least not many block changes/renames between 1.14 and 1.15. So the rebuilt and saved schematic from 1.15 should work fine in 1.14 as well.

It will print any unknown block states to the game console when you click on a schematic in the file browser GUIs or otherwise load a schematic from file, that way you can see if there were any renames and thus unknown blocks (converted to air) from going between 1.14 and 1.15.