There are already land leases which go beyond 2047 (I own one such property) and, to date, the HKSAR Govt has made a practice of renewing expired leases for non-renewable terms of 50 years even if that pushes beyond 2047. However the sample size is small (I think).
There's a good summary in the linked Legco webpage. Key points:
1. it's at the government's absolute discretion but they have exercised it in at least some cases and no fees have been charged when they have done so. (Just my opinion of course, but I seriously doubt that the HKSAR govt would do that without the blessing of Beijing)
2. as far as I am aware they have not made it a matter of policy to automatically renew all leases BUT
3. I'm not aware of any cases where the government has not granted a 50 year renewal on expiry – if there are any, I would very much like to know.
4. as far as 2047 is concerned, Article 5 of the Basic Law provides that "The socialist system and policies shall not be practised in the Hong Kong Special Administrative Region, and the previous capitalist system and way of life shall remain unchanged for 50 years". It does not automatically come to an end then but the PRC government has the right/power to decide what happens.