There is a world of difference between “once a Democrat” (which I btw was) and a Democrat.
Some Democrats are more conservative than the Country Club Republicans that plague the GOP today.
But I will admit that I do not know enough about the General to do more than speculate. (I suspect that most of the retired brass that Obama has forced into early retirements, will be all in for Trump anyway--probably with hair raising narratives to offer as the campaign unwinds.)
There can be but it is no guarantee. There are a lot of former Democrats in the Green Party and the Progressive Party.
Given his earlier position, he would be uniquely qualified to call BS on Hillary's claims concerning her handling of sensitive materials. Provided he doesn't have a skeleton of a similar nature in his closet. Hopefully, that is being carefully vetted.