That's an interesting question, because they do teach that one has to be made a spirit before one can attain heaven.
But, what's more interesting, at least to me, is that there seems to be a set time for the marriage of the lamb, as it's called in Revelation, or when the "bride" or the church/congregation is made one with Christ in heaven.
According to WTS doctrine, that is only the few annointed of course, no one else is born of spirit or spirit annointed.
But, if theyh're on earth, don't they miss the wedding? Because in their timetable, it seems to occur at a specific time, after Armageddon. (I've even read some things where it seems to occur just before, again, sort of a rapture of these ones only, but that was some time ago...60s, 70s?)
Admittedly, this is a bit of WTS doctrine that goes back and forth. It was originally though that the remnant of the 144,000 still would join with those already in heaven since the invisible presence of the Lord in sort of a "rapture", but then it came to be thought that some of them would remain on earth through the great tribulation, Armageddon and then either die naturally of old age or be taken, raptured in effect, giving up the body, sometime in the New World.
The thought of giving up the flesh was always there, though. I suppose the thought is that they would not be given the same provision of returning to youth and endless human life like those who embrace that. How cruel! To continue to grow old and die when all around you are becoming young again!
The "sudden death" idea seems more humane, as it's still thought that one has to give up the flesh to attain heaven, but again, death is death. I suppose they would think it would be quick and painless, like Enoch.
But, you see the torturousness of WTS doctrines here. To keep that two class system in effect, they have to make all kinds of twisted speculations on these things. Also, whether resurrected humans on earth would have normal sexual and family feelings. Then they have to deal with the speculation by dictating they ignore it.
I think what the scriptures actually say on the matter is so much less complicated!