discipline conversion

I have heard that horses who have changed disciplines have less chance of being placed in the first 3 placings of shows.  If this is true - how long should I keep the horse in the riding school before entering further shows. The horse in question is a Chincoteague foundation mare who is nearing 16 with no prior foals and I have changed her from driving to endurance.
Many thanks for your help
kind regards

I dont think its true that horses who convert disciplines have less a chance of placing, i think it depends on what you convert your horse to and what their base stats were mixed with what their non specialty stats are after converting. My entire division of trakehners got messed up when i continued feeding them hay cubes instead of speciality related treats, so where I originally had them trained in dressage, i converted them all to show jumping, and because they were foundation horses their non specialty stats dropped back down to a healthy number and all other stats went straight into speed and agility. Those horses now place 1-3 in almost all their shows every single day, Nike and Prometheus being the greatest examples. 

Who is the exact mare in question?

I've never heard of this and have no way to know if it's true since I rarely convert, but I don't think conversion will affect your show placings unless you're converting a horse to a discipline it's not suited for. For example, a Show Jumping bred Warmblood that you convert to Dressage would probably do poorer in shows because it would have a higher speed component.

If it is Secrets in the Myst you refer too it could just be she has been unlucky to be competing against other horses with lower non spec stats. I tried looking at her show record to get more details on her show ability but it wouldnt let me. Even a few less non specs can make a significant difference IE a rivals 55 to her 58 & anything over 60 non spec is a waste of show entry fee!

I find some horses though just dont show well in general for some inexplicable reason & these I just put in the Riding school.

I dont convert often but on a few occasions I Have & mostly didnt have any problems show wise except one driving friesian who had massive non spec stats before I bought him & a convert training improved them hugely but still not enough to show well. 

Thank you all for your comments.  I will enter some competitions and advise on the results.