The simple answer is you can't. There are incompatibilities between these versions that cause items/signs/other data to be erased upon downgrade. There may be some other way to do it that I am unaware of, but I would imagine it would easily risk data loss.
How do I downgrade Minecraft from 1.8 to 1.7.10 without loosing all of my items in my inventory and chests?
My anaconda will consider it.
The simple answer is you can't. There are incompatibilities between these versions that cause items/signs/other data to be erased upon downgrade. There may be some other way to do it that I am unaware of, but I would imagine it would easily risk data loss.
Then is there a way to use an inventory editor to copy items from a copy of my world running 1.8 to a copy running 1.7.10?
My anaconda will consider it.
You could try, but even so, you still run the risk of losing world data. Make sure to make a backup first before you attempt this.
For anyone that finds this thread and needs the answer, I this is what I did.
1. Turn anything you can into blocks (Iron Ingots --> Block of Iron)
2. Place those blocks somewhere in your world
3. Make a copy of your world
4. Rename the copy as [world name] 1.7.10
5. Rename the original as [world name] 1.8
6. Rename your Forge Installation as Forge 1.8
7. Install Forge for 1.7.10 (Do NOT rename this one)
8. Open [world name] 1.8 without Forge in a window (not fullscreen)
9. Open your inventory
10. Alt-Tab out of Minecraft
9. Download and open this program
10. Open the [world name] 1.7.10 save
11. Recreate your inventory
12. Click "Save to [world name] 1.7.10"
13. Done
Edit: Oops I forgot to link the program
My anaconda will consider it.