650C wheels are 51mm smaller in diameter compared with 700C, so the front end of your bike will be about 25mm higher than was intended. Worse, the entire geometry of the bike will be skewed - both head and seat angles will be much softer, and the bottom bracket will be appreciably higher, for example.
If the bike's been built for 650, then that's basically what you need to stick with in my opinion.
Whilst i agree with that ,would the front end not be the entire 51mm higher ?
I did replace a MTB 26in suspension fork with a 29er rigid fork the other year and it feels fine to be honest .
I also replaced the original fork on my Y-foil TT bike (rock shox ready) with a shorter pure road Look one that steepened the head and seat angle a tad and it was fine too
Yes, that was quite common back in the days when front suspension was just becoming popular (or, at least, the reverse was popular!). I think the effects on a TT bike of such an extreme change will be more marked, though.