Profit Max,
we are always talking here of the RA as a (i.e. THE!) last resort. As such we are talking of a matter of seconds. If a once given advisory was changed this would probably cause more confusion than help - especially when we continue toying with ideas and think of a situation where the crew who had not complied in the first place (here: the TU154) suddenly make up their mind and start following the (original) RA (not an unlikely scenario, as we know now that a short discussion about the RA had taken place in the TU154 cockpit). This would then create yet another RA, and so on, rendering TCAS unnecessary/useless.