@knallbär Great Question!!!
This is a very hot topic, and I would not be in the position to say which way is the best way, however I can provide information as to the three most common moves that I have seen and both pros and cons of those choices.
Move all 20 units back to Moscow: 6 infantries can make Moscow by Turn 6 while the other 12 infantries and 2 AA guns can make it on Turn 7. This means 6 infantries make it before the Germans make what is their normal Turn 7 attack on Moscow and if Moscow holds all units can make it for the defense of Moscow.
Pro #1: This provides the most short-term protection of Moscow. These 20 extra units allow Moscow to hold to Turn 11 or 12 versus the normal 7 or 8. Those 20 units are worth 64 TUV.
Con #1: With those units gone Japan will most likely start attacking the East Russian Coast (Siberia) on Turn 2 or 3 at the latest. Going for Amur on about Turn 4 or 5. Rolling up the east flank getting to Yenisey by Turn 6 or 7. This costs Russia about $22 by J7 which means Russia loses out on 7 infantry (and thus only really gains 11 infantry and 2 AA Guns) and Japan gains that same $22. Going forward each Turn costs Russia $7 while Japan gains $7.
Con #2: With no Russian troops in Amur Japan is free to send all ground troops against China greatly improving their China attack.
Leave all 20 units in the East: Leaving all 20 units in the East, usually going to Buryatia on R1 and Amur on R2, puts pressure on Japan to defend their northern flank and protects the eastern Russian front.
Pro #1: Forces Japan to put 10 infantry, AA gun and 2 fighters in Korea to protect Korea from a Russian invasion. Those troops are badly needed for the attack on China and thus without them China will hold longer and collect more of their own monies.
Pro #2: Russia does not lose any IPCs due to a Japanese attack so in the long run they collect more money.
Con #1: Without the eastern troops Moscow is more at risk of falling and if Germany cannot take them on G7 they have time to build up for a G8 or G9 take since no additional troops are coming.
Move 6 infantry back to Moscow: This compromise provides more defense for Moscow before the normal German Turn 7 attack, 6 infantry, and still protects the bulk of the Russian eastern money though fewer Japanese troops are needed for the defense of Korea so more pressure is put on China. Those 12+2 units are probably not strong enough to stay in Amur and will linger in Buryatia waiting to move to Amur if Japan leaves Manchuria too weak. Being in Buryatia allows Japan to take a little Russian territory.
In summary: Bringing the troops back is more of a short-term solution and favors the Europe side of the board. It takes Germany longer to take Moscow but at the same time makes Japan stronger. Leaving the troops in the east weakens Japan significantly but puts Moscow more at risk, though Russia will collect more money until Moscow falls. Bringing back 6 infantry is a compromise that still protects Russia’s money however Japan is able to free up more troops going against China.
So, what is more important to you? Do you want to hold Moscow longer? Do you want to make Japan easier to control? Really that depends on your overall strategy and where you are putting your emphasis of attack.
Hope that helps…