Hi to all,
Skipping grade is a good thing, but if you are to do so, a lot of time and effort has to be put in to practicing. Some of the biggest changes that you will most certainly feel if you are skipping from say grade 5 to 7 or 8 would be in: the sight-reading, technical requirements and AURAL.
If often feel that when people skip grades, the Aural aspect is often not addressed as well as the pieces. So, it is important that if you decide to skip a grade that you know what you are doing and that you feel confident in your answers, especially in the general aural part at the end of the aural tests.
Unless your Sight-reading is very very good, you might struggle in achieving the requirments for a pass at higher level sight-reading. However, sight-reading requires practice, and as long as you practice upto and including the standard of the grade, you should be fine.
As far as scales go, tempo plays an important role, so make sure you are playing at the tempi indicated in the scales books.
If you can do these, skipping a grade won't seem like to much of a leap. Without such preparations, you could struggle and find yourself taking the higher grade at a time that you might have been taking it if you were not to skip grade.
Skipping grades is possible, my first organ exam was grade 7, and that was only having learnt for one year!
Hope this helps,

and sorry its so long.