I think they transfer reward points to your upgraded card but on a proportionate basis. Say, you have 50k points on your card which you earned @ 4RP per Rs 150 spent but with the new card, you will earn @ 5RP per Rs 150 spent. Then, they will credit your new Card account with 40k points.
I was upgraded from some basic card to an allmiles card in 2015 and then to regalia in 2017 but never lost any RPs.
But, better talk with customer care before hand preferably over email, to avoid surprises.
Thanks,