If you just played CMBO, you will have to bridge two gaps. The first is big. It's the gap between CMBO and CMBB/AK. It's mostly about the correct simulation of infantry weapons.
The second is huuuge. It's about new engine, more detail, more realism etc. It took a year for me before I was used to CMSF. But I also had to understand modern warfare first.
I would recommend to practice at least with the CMSF demo to learn all the new engine basics. Especially if you plan to play pbem (which is the 'grand' game mode for CM games, because here all the wargamers of the planet meet at the virtual battlefields. But you have to know what you are doing, if you want to have at least the chance of a chance). You don't need to buy CMSF + the moduls, if you are not interested in modern tactics at all. But you could do it nevertheless to support battlefront for producing the best tactical WW2 wargames we will see in the next years.
I personally found out for me, that I even like the modern setting more, because it is more unpredictable, more deadly and more complicated. I would say, those who have mastered CMSF + it's modules to the point to be able to win 50% of their pbem against human opponents, will have it easier to win their coming CMx2 WW2 pbem against 'only WW2 wargamers'.