That's how it should go, or more like all the old stuff is "imported" into Arrowhead. Remember that with the engine everything is "mods", packed in .pbos, which the game loads when it starts. You could even remove all the original .pbos and it'd still run. I still don't have Arrowhead thanks to the great summer weather we have(no time for gaming) so I can't say for sure, but that's my impression from reading
BIS forums.
Anyone new to the series should go for Combined Ops, as it has all the content published so far(Arma2+Arrowhead).