I think another good idea when considering trainer changes is to decide whether both the old and new trainer have had a good season or not.
As an example, the 9.00 race at Kempton last Wednesday.
The horse is Bankrolled.
Left Richard Fahey and gone to Jane Chapple-Hyam.
Richard Fahey has had a pretty disappointing season in general, (although I consider him to be a very good trainer).
Whilst Jane Chapple-Hyam has had a pretty good season.
Richard Fahey ran Bankrolled in a couple of big sales races last season as a 2 year old, but this season has only been able to get a distant 5th, (albeit in a class 4 0-85 handicap), at Haydock.
First run for new yard after a five week or so break, came 2nd in this Kempton race at SP of 12/1, (in a class 6 0-65). Only beaten by a horse that made all for the first time (Pinball Wizard). The winner has been criticised as a very tricky horse, but looked very straightforward in this race, with the change of tactics seeming to make a big difference. He also had amongst the best form in this weak race anyway.
Main point being: that I expect Bankrolled to be winning soon, and in all likelihood to be rated higher than 59.
Think the headline angle with trainer changes is often the strike-rate achieved by trainers with horses making their debut for the yard. Whereas, I think the key thing is whether the new yard will improve the horse, (whether that be compared to the horse's career-best form or best form from the previous yard). Very few trainers achieve a strike-rate of above 17 or 18% on debut for their yard anyway.
Of course, you then have to work out when the yard will improve the horse. Things are rarely easy when betting on horses!