Maybe the reason for this change of strategy is the hope that Trump might not have a majority of delegates onside come November and thus a brokered deal could be made to foist Cruz or Rubio as the official nominee? Weird stuff.
This scenario has been sketched out already and there is a growing likelihood that its now the adopted game plan
There are a number of issues
In the first case Trump signed a pledge not to run a third party campaign and in return the GOP agreed to provide a level playing field and a fair fight. The GOP are widely held to be reneging on this. If Trump wins a plurality of the vote, and the convention then swindle him out of in June, it'll be too late to get his name on the ballot. Many states also have something called 'sore loser' legislation which the GOP would equally use to block his nomination.
The GOP's bigger fear is if Trump sits in television studios I believe bad mouthing the whole process, reminding everyone of the flawed democratic process and using the media as a platform to vindicate everything he alleged at the outset about the corrupt American system whereby a handful of donors and large corporate interests dictate the party line. The GOP can probably absorb another White House loss, but they fear the damage that would be done 'down ticket' in all the other races that Americans vote for on the same day. There is a reason why Mitch McConnell is considering running attack ads against Trump, as he needs to separate the senators from association with the Trump party
The other possible scenario emerging is the GOP running a rebel candidate against Trump. Crazy at face value, but by no means out of the question. Doing this hands Clinton the White House, but possibly protects the Senate and Congress
America hasn't really come to terms with it yet so far as I can see. They haven't recognised that Hillary is actually their last line of defence, or will be in a matter of a month on this current trajectory. If they don't rally behind her, America stands on the cusp of electing someone to whom the word fascist could be applied and not be entirely inappropriate
Heads are spinnin'