Transfer between 1.16.5 to 1.12.2

You can't and shouldn't anyways. Rule of them for making mods, you can also start low and go high, but usually cannot start high and go lower. At least not easily.

As HookerHeels said. Transferring back is not possible without plugin, and using plugin, you would have more errors to fix than actual working workspace.
My suggestion is to start new workspace on MCreator 2020.2 and just lurk to your old workspace to see how you did some things (so it's just "look and rewrite" work). Remember though that MCR 2020.2 was really limited in terms of features. This is why you would have a lot of errors if you would want to just "transfer back".

"As HookerHeels said"

Priceless haha

Kyufey, even updating from 1.12 to 1.16 is almost impossible depending on the mod. Anyways 1.16 is the new 1.12 IMO.

Chew

Update from 1.12 to 1.16 is possible, but it is best to do it the "slower" way:
2020.2 (1.14) -> 2020.3 -> 2020.5 (1.15) -> 2021.1 -> 2021.2 (1.16)
This way you will have as little bugs as possible, but it's tedious thing to do. If you do 2020.2 -> 2021.2 migration, it will most probably not work.

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As for 2020.2, make sure you have old version installed in different folder, and that you have Java 8 installed in system too. You can also try to regenerate the code, it fixes some errors related to Gradle sometimes.