Your old motor was 1.85 total and it came with a SPX3210C impeller. That shows what hp the impeller takes. You never reduce hp on a pump with original parts without reducing the impeller.
You have to go to Hayward's TriStar page to see the models and what total hp each has. Then, you go to parts to see which parts each takes.
The 1 hp full rated pump takes a SPX3210C impeller and uses a 1.85 total hp motor.
The 1.5 hp max rated tristar takes the same SPX3210C impeller and uses a 1.65 total hp motor.
Your original motor was 1.85 hp total, so that's normally what you would need to use unless you replaced the impeller.
However, since the max rated tristar uses a 1.65 total hp motor with a SPX3210C, that means you can use a 1.65 hp motor.
The SPX3210C will overload a 1.25 total hp motor and cause it to fail earlier than normal and it might overheat enough to cause the thermal overload to turn the pump off.
The reason that the flow is the same as before is because the impeller is the same.
It would be best to switch to a SPX3207CM or SPX3205C impeller.
I can't seem to find this info on the Hayward site. Could you give me an exact link?