I have Futaba skysport 4ch 35 Mhz. I want to switch to 2.4 Ghz by using 2.4 Ghz hack like assan or frsky.
I think Assan Hack in india will cost me around 6000-7000 Rs and at same price I can get any cheap 6-7ch 2.4Ghz radio.
Now my question is what is advantage of using hack over old FM radio than purchasing new cheap 2.4 Ghz radio...?

I would suggest that you go for a cheap 2.4Ghz radio.  The main reason is that these newer radios have so much more functionality, including the bare minimum REQUIRED features (like multiple model memory, channel reversals etc).

If your radio were something better than the Skyport, then the thought process would have been different !

Is there any advantage of using hack module other than no interference...?
Do we have same reliability as futaba after converting from fm to 2.4 Ghz with assan or any other module...?

The Assan hack seems to be a proven solution (based on what is reported so far on this forum and else where).  Not sure if people would put their $3K turbines on it, and there were some CDI interference issues reported on gassers. But for normal use (ie not at the highest end), they are just fine.

The advantage of using a hack module is the advantage of 2.4Ghz (no conflict from other flyers, more resistant to environmental interferences etc), which is now made available on a regular Mhz radio. In terms of reliability, I would rank the common ones as Airtronics, Futaba, Spektrum/JR, Assan, Other Cheap 2.4Ghz radios (in the decreasing order of reliability).