Well, in this one only the French medium line will be moving, AMX 12t and up will all get bumped up a place. This means that you want to get two tanks that are atleast two tiers apart if you want to get the maximum number of slots. This is ofcourse if I didn't mess the numbers up and they do it the expected way.
If I'm correct it should go like this:
You have an AMX 12t and nothing else, you get a second T5 light while the AMX becomes T6
You have the AMX 12t and the 13 75, you get the second T5 light while both AMX's get bumped up a tier
Same as above if you have all the other tanks.
BUT!!!!
If you have the AMX 12t, and say an AMX 13 90, or a Lorraine 40t or the Bat.Chat. you would get a second T5, the AMX would go up a tier and you would also get a second tank at either T7, T8, or T9.
In the extreme case of you having the AMX12t, the AMX 13 90 and the Bat.Chat. you would get three extra tanks, and with them you would have the complete French medium tree from T5 upwards.
Now, none of this is certain untill the devs confirm it (or someone from the test server who expirienced the switch in these exact circuimstances), but the least you can do and that will be the most certain of getting you atleast one slot is buying the AMX12t again. Right, scratch that, official info I guess:
Case 1. AMX 12t isn’t in the garage at the time of the update release, but is fully researched:
Both tanks, ELC AMX and AMX 12t, will be researched
All undistributed experience goes from AMX 12t will be moved to the ELC AMX
Any crew member in the barracks or in a tank, who possesses the tier 5 AMX 12t skills, get the ability to be used in the tier 6 AMX 12t without any changes or losses
Tier 5 AMX 12t combat statistics will be moved to the tier 5 AMX 12t without any losses
All AMX 12t modules in the depot that don’t fit other tanks will be automatically sold at the original purchase price
All AMX 12t modules fitting other tanks will be saved in the depot, the same goes for shells of the respective guns
Case 2. AMX 12t is in the garage at the time of the update release. All the rules as mentioned in Case 1 apply along with the following:
AMX 12t as tier 5 light tank is substituted with AMX 12t as tier 6 light tank
Ammunition mounted onto the AMX 12t goes to the depot
Consumables and Equipment mounted onto the tier 5 AMX 12t remain on AMX 12t at tier 6.
Players get the light french Tier 5 tank ELC AMX (without equipment, consumables, ammunition) and one slot for this tank for free
Players get a new full crew for the ELC AMX with 100% training
Camouflage mounted onto the Tier 5 AMX 12t is moved to tier 6 AMX 12t and to ELC AMX as well
The expiration date of the camo on both vehicles stays the same as it was for the AMX 12t at the moment when the substitution takes place.
So the only reason to get any of the higher tanks (13 75 and 13 90) is because they are going on sale this weekend, and if you wait for the patch and then sell them you would make some cash, other than that no change worth mentioning.