Known factory upgrades :

Pz-IIf    > Pz-IVg      > Pz-IVh  > Pz-IVj
Czech38t  > Czech38t(e) > Pz-IVh  > Pz-IVj
Pz-IIIg   > Pz-IIIh     > Pz-IIIj > Pz-IIIm > Stug-IIIg
Pz-IVe    > Pz-IVg      > Pz-IVh  > Pz-IVj
Tiger     > Knigstiger
Stug-IIIb > Stug-IIIg
PzJg-Ib   > Marder-III  > Elefant > Nashorn > Hetzer
Marder-II > Hetzer

Me-109E     > Me-109F   > Me-109G > Me-109K
Fw-190A     > Fw-190D
Me-110C/D/E > Me-110F/G 
Ju-87B      > Ju-87D    > Fw-190F
Do-17Z      > Do-217    > He-177

T60         > T70       > SU-76
T26/BT-7    > T34 M40   > T34 M43 > T34/85
KV-1 M41    > KV-1E     > KV-1S   > KV-85  > JS-IIM > JS-III 
KV-II       > SU-152    > JSU-152
JS-I        > JS-II M   > JS-III
SU-85       > SU-100
SU-122      > JSU-122

i15    > Hurricane II > Spitfire IX  >>> not possible to change factory back
i16    > Yak-1      > Yak-7b  > Yak-9  > Yak-3 > Yak-9U
Mig-1  > Mig-3      > Yak-7b  > Yak-9  > Yak-3 > Yak-9U
LaGG-3 > LaGG-3 s33 > La-5    > La-5FN > La-7
SU-2   > IL-2       > IL-2/M3 > IL-10
Pe-2   > TU-2
SB-2   > IL-4       > TU-2
