I don't believe that Trump is relying on the party line voters anyway, he is aware that they are not accustomed to an America first position and would rather have a softer candidate.
Given that, and the fact that out of the 142 million registered voters in the US in November 2014, only 92 million voted, the remaining 50 million voters, which I would classify as independents.
Trumps likely focus is on that group and, as they have no party addiction, would follow Trump to any party he moved if he had to.
But we shall see...