score:6
Preamble: Youβre not entitled to a UK Visitor visa, nobody is.
Now to answer your questions:
Is there any legal way to change my visa other than reapplication?
No
If I reapply for a longer visa , do they have the authority to cancel my existing visa ?
Yes
And if they refused my application for a longer stay , will this be counted as a second visa refusal which could affect my future visa applications ?
Definitely.
Consider yourself lucky you were approved after the earlier refusal. My advice is donβt push your luck