if you change them at 50k , you are doing it at the right time.. coz you dont have to wait for one of the tyres to burst to get new ones...you have had the best of them... and you they will still fetch you 500 rs per tyre atleast..when you go in for new ones...
if the tread is ..ok .. and soft
the sidewalls hav not got cracks (ozone cracks)
and they are not bent...
you can still manage 20 more k...
with the loss of reliability, compromise in handling, and chances of burst
but you should keep them properly inflated.. coz the sidewalls are weaker now...
i have got dunlop sp70 on my car and they have done more than 60 k.. tread is still good.. n soft, tyres r not bent... but there are sum ozone cracks developing in the sidewalls...