We had exactly this problem when my child transferred (in 11th grade) from a more rigorous school to a school that was in fact less rigorous but which believed there was no more rigorous school in the world. (For the most part, it was right. It was traditionally the main academic magnet school in a large urban school district. Most people who transferred in were transferring from less academic schools.)
The two schools had completely different grading cultures. The new school had honors classes and AP classes with different weights. The old school had no honors or AP classes. The new school had grade inflation and used a 100-point scale. After weighting, 25% of the class had a GPA higher than 100. The old school used letter grades, and an A- average meant you were in the top 10% of the class and likely to get early admission to an Ivy League college or top LAC. The new school had 600 students per class and a median SAT of 1100. The old school had 95 students per class and a median SAT of 1320. That said, the best students at the new school were absolutely the equals of the best students at the old school, except more STEM-oriented.
There was a battle with the new school over how they translated the old school’s grades, and how they ranked her, but it was a battle we largely lost. They said, “Don’t worry, by the time she graduates she’ll be in the top 10%,” and I said, “Why should I care where she is when she graduates? When she applies ED to college, you are going to be saying that she isn’t even among the top 100 students at the school.” Then she was one of only three National Merit Semifinalists in her class, and the school administration – which cared a lot about how many students got into prestigious colleges – realized that they had not acted in their own self-interest, but by then it was too late.
When she applied to college: (a) She supplied the original transcript from her old school. The old school was familiar to the admissions staffs at the colleges to which she was applying, and she thought they could see that her transcript there was the transcript of a student who was competing at the top of her class, not someone in the middle of the pack. (b) She got an additional recommendation from one of her teachers at the old school, who addressed her curriculum there. © We negotiated specific language that the principal would instruct the GC to put in his school report to the effect that the way the school had imported her former school’s grades did not necessarily reflect her performance at the former school and was not consistent with her performance at the new school. (But we have no idea whether the language ever actually got into the report. The GC didn’t want to say anything like that, which is why we were talking to the principal about it.)
In the end, did it hurt her to change schools? From the standpoint of education and personal growth, no, it was great. From the standpoint of college admissions, who can tell?, but probably. She was 0-2 at the super-selective colleges she most wanted to attend. But she had a great, realistic strategy, and everything worked out fine for her.