"In the 'Learn the dances' section, the 'man's part' etc lists the steps to be followed... However, it does not mention the most important part ie. How will the man SIGNAL the woman that the step will change
eg. simply put, how will man signal the change of step from basic to promenade in Tango"
Mostly by dancing his own part correctly. When the couple is closely coordinated, there's enough information implied in the man's actions to make the lady's complementary actions fairly evident.
It can be usefull to identify which specific component of the man's action triggers which response from the lady. Unfortunatley, the most common american style promenade opening employs a fairly subtle rotation of the hip to the right which requires great skill. In contrast, the most common international style opening (link) is quite clear and simple - the man places his left foot diagonally so it is in front of his right (will feel like it's behind the lady but actually is not) and then neutralizes his body to that - this puts the couple in promenade.