The rules don't say replace the 2Ts with the 2T/1Ms it is kind of vague. How do we know this?
If at least one Minor Company was formed in the first Stock Round then add one ‘2’ train, one ‘2M’ train and one ‘3M’ train to the train mix. The seven type ‘2’ train cards are available to all companies but for Minor Companies only they are now dual-option and can be purchased as either a ‘2’ train or a ‘1M’ train. (REF 2.8.4). If this rule is applicable because a Minor was formed in the first Stock Round, then all seven of the dual-option ‘2/1M’ trains must be purchased before the first ‘3’ train may be bought.
I think the description of the 7 trains as "dual-option" is quite clear.