It seems to me that the urge in primary TS comes from the need to get away from their male selves as soon as possible, whereas in secondary TS the urge comes from feeling life is slipping by and your window to transition into a "good looking" woman is closing rapidly.
So long as the state of mind is correct, ie a realistic evaluation that you are simply trading one gender role for another with a different set of problems but that this will make you happier, then it does not really matter when or how you reach that point, transitioning (whether full time of SRS) is the correct path to follow. The danger is in rushing into a decision before you are ready for it.
The question as to whether a CD changes into a TS or if the person was always a TS hiding as a CD, is a chicken and egg story. Until the point of realization the CD will think that he is just a CD with TS fantasies, but after he makes his decision, he will believe he has always been a TS but just afraid to act upon his emotions.
The truth probably lies inbetween. Clearly by deciding upon transitioning in mid life, their gender dysphoria was not that explicit and they lived well enough as a male so there must have been some movement during the years. I think that secondary TS were probably always transgenderists - lying inbetween crossdressers and transsexuals and so had a foot in either camp and so could decide either way. Since we only have one shot of living this life, the desire to try out the alternative is fairly powerful.
A TG has a realistic viewpoint on changing gender not the fantasy wishes of CDs although I suspect that is not the case at all during childhood/teenage years. The TG component in the transgender spectrum is overshadowed by the CD and TS and is not discussed much but I suspect that many secondary TS are to be found there but label themselves as CD.