"Models that were engaged with just one of the enemy units at the beginning of the combat (before any model attacked) must attack that unit.
5th Ed FAQ
Page 41 – Multiple Combats, Attacking.
A third bullet point should be added, as follows:
• Models that at the beginning of the combat
(before any model attacked) were engaged with
more than one enemy unit, but were in base
contact with just one of the enemy units, must
attack that unit.
I'm reading those two and interpreting them differently. What I'm reading is that I must attack models in Base to Base contact. I'm not seeing anything in those rules that would deny someone the ability to attack a new attacking unit that is also in base to base. I tried bolding what lead me to said conclusion.