[stormbringer3]

In the past, whenever I played long campaigns, I always used a lot of infantry units. I usually bought for my core units what was best to use in the game without a lot of concern for historical accuracy in the unit mixes. Now I want to try the campaigns with a more historically accurate unit mix.
When I purchase my 1939 infantry formations will any units be able to be upgraded to assault gun vehicles in later years? If I remember correctly you can only upgrade AFVS to AFVS, and not for example, infantry gun units to assault gun units. I know I can always buy assault guns as new units for my core when they become available, but I was hoping to be able to upgrade an experienced unit which I started in my 1939 core. I don't remember any assault guns being available in 1939. My new game will arrive in a few days so I am trying to refresh my memory on this.
Thanks for any info.