i think you are too, arnav is not a person to change for anyone but for his sister... he might change for khushi but to see that day is a long time... there has indeed been a drastic change in personality of the arnav after the dance.. i think that arnav saw the misbehave done by shyam to khushi and he tried to woo khushi to gain her trust like u said and ultimately marry her...
it has been indeed mentioned many times that arnav is willing to give up anything and do anything for his sister. Moreover at times, the concern of arnav for anjali is shown like after the scorpion incident. he didn't care about shyam all he had in mind was anjali...
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