The ability to convert Premium Days into 3Hour Credit Boosters, cost for Credit Boosters will vary based on the percent boost you will get.
1 Day of Premium = One 25% 3Hour Credit Booster
3 Or 2 Days of Premium = One 50% 3Hour Credit Boosters
7 Or 5 Days of Premium = One 100% 3Hour Credit Booster
12 Or 10 Days of Premium = One 200% 3Hour Credit Booster
The ability to convert Premium Days into Credits or Gold.
1 Day of Premium = 100,000 Credits
1 Day of Premium = 25 Gold
The ability to buy Premium Tanks using Premium Time.
The conversion value for Premium Tanks will be either 1.5 to 3 times the Gold value equivalent of Premium Days.
Special Premium Tanks that you can only get by using Premium Days to get.
And finally the ability to use them as a resource in the Assembly Shop if they ever plan on doing a similar event again.
3 Days of Premium = 1% Assembly
You may notice that most of the most of the ways of spending Premium Days I listed aren't economy friendly to the premium day spenders. I set it up this way because I rather have it discouraged normal to players from using this from of in-game currency conversion. We all know that most players can earn up to 100K+ Credits in a hour with premium time, really good players will earn up to 500K+ Credits in hour with premium. Witch would make about all of these uses for premium days trade not worth it. Most Whales have over 500+ Days of Premium, I can't even think of the last time I ever needed to buy just Premium Time. All of the times I get Premium Days in a bundle with a tank, and the only reason why I even got it was for the 5x exp multiplier for wins in battles. So these ways of spending Premium Days would give whales a reason to buy just Premium Time, I play for 1-3 hours a day so I feel like spending Premium Days as a currency for credit boosters sounds like a deal to me.
Plus life is full of changes you might not even live to use up all of your own premium time in game.