Source: Russian support site (via Loyolla from Ponyushnya, thank you)

Please note that this policy seems to apply only to bonus codes specifically.

Hello everyone,

looks like an interesting policy change took place on RU server (given the nature of the change, it’s fully expected to happen on other servers as well).

Basically, if you have a premium tank in your garage and get another one (for example by bonus code, or you win it in some event like the TOG we had not long ago), you will not be compensated with the tank’s price in gold anymore. Instead, you will recieve credits, using the ratio of 1:400.

An example: if you have a tank worth 1000 gold in your garage and get another one (for example by bonus code), you will instead of 1000 gold now get 400k credits. But if you bought the 1000 gold tank again in the premium shop, you’d get 1000 gold.

Discounts still apply as before, eg. if you were to get the 1000 gold mentioned in the example above and the tank was on 50 percent discount from an event, a bundle and such, you would get only 50 percent compensation (eg. 200k).