Dave Warner bats both ways. He doesn't see why he should have to stick to one or the other during the course of the match, and there certainly isn't anything explicit in the laws to say he should. If he switches after the ball is in play, that is once the bowler's run-up has begun, it's all confirmed as fine - he is officially batting one way, and takes on the risk of changing in a restricted time-frame for the benefits of catching the bowler and fielders ready for a different approach. (Of course, it might rule out the leg-side wide and open up LBWs as well, but I wouldn't want to guess what the MCC are doing with this in this year's planned update of the Laws.)
If he changes just before the run-up starts, then he is officially batting the other way. Either the bowler is caught bowling to different rules and fielders are potentially illegally placed (not to mention tactically undone), or they notice, and will stop and take the time to change the field. Clearly against the spirit of the game, as the umpire told him.
I guess it's clear what these umpires would have done in February 1981. However, I'm not sure he should have been stopped from doing it in general. Yes, batsmen switching over adds one more time delay factor to the game, but is it any less justified than changing the field after each single for many partnerships? In general terms, if switch-hitting is ok, surely batting one way or the other with notice is in the spirit of the game! There would be no objection to batting left handed in one innings and right handed in the next, so where do we draw the line?
Mind you, why change anything when you are making 50s in 18 or 19 balls? I don't know whether the hype about this innings was going to last any longer than a T20 game in the first place, but it has certainly gone missing in the light of a more recent innings. Tendulkar's 200 was enough to excite even this fan of lower scoring matches!