Since premiums generally don't go above tier 8 let's say the feature would only be for tier 8 and below tech tree tanks.
There are plenty of really fun tech tree tanks that I prefer playing compared to their premium version. For example I would prefer playing the VK100 over the Mauerbrecher, but I would also like to have the credit income and crew training of a premium.
To solve this I propose that players should be able to turn their tech tree tanks into premiums with a one-time gold fee. So how expensive should it be? WG still wants to make money so it wouldn't be unfair to say it should be equal to their premium equivalent. In that case the VK100 would cost about 10k gold to turn into a premium.
Some would probably think it's too expensive to buy that but that's fine. Others would be willing to spend and they'd get a nice credit buff to their favourite tech tree tanks and WG would surely make more money since people would be more likely to convert the VK into a premium than to buy a Mauerbrecher currently.
