IOTL a couple of nations such as Pakistan did show an interest in the Upholders once the RN decided to dispense with them. The boats were still very young, very modern and going cheap and at the time were laid up awaiting a buyer. Part of the cause of the issues that the class experienced OTL was due to them having spent too long laid up deteriorating while Canada tried to make up its mind about acquiring them.I think not because (with the exception of Canada) nobody was buying British submarines at the time. E.g. the Australians replaced their Oberons with a Swedish design and the South Africans eventually replaced their Daphnes with German Type 209s. I suspect that the only reason why Canada bought the Upholder class was that they were "going cheap" due to becoming surplus to British requirements.
If the SAN was already operating British built Oberon's they would have likely looked at possibly replacing them with Upholders.
When the possibility of acquiring brand new surplus Upholders arises with the added bonus of the boats being cheaper and available immediately compared to new builds I'd imagine that a South African Navy already operating British boats would have given some serious thought to making an offer and even perhaps ending up in a bidding war with Canada.