Was R4 and R6 values changed? These are usually changed in pairs to maintain hystersis. Also the new turn on voltage must be set higher than the LED string voltage otherwise the output will go to maximum duty cycle before the LEDs conduct which can cause flicker. What are the value changes and again what is the PWM frequency?
The values of R4 an R6 should be calculated using the TPS92510 spreadsheet available on the WEB. If the string voltage is 19 volts the turn on voltage should be calculated for a turn on of 21 to 22 vdc.