Most cards that come with the units for both versions (1 & 2) have been revised. Print the revised cards and everthing is playable together.
A good amount of units from version 1 have been re-released in version 2. There was no scale for vehicles in V1, some units are almost the same as V2, some are wayyyyyy smaller.
V2 has bigger hexes to hold the larger size vehicles but both version’s maps work.
There are several different rules, I suggest to print the new ones, they are alot different.