They have always believed in having an open and accountable government. In an ideal world, there would be civil societies and activists to give pressure on the government when they do something wrong. Unfortunately, during the 1980s and 1990s, civil societies and activists in Singapore are non-existent, and SDP at that time had seen actions by the government then that they felt was wrong. With the absence of civil societies, and despite knowing that it is to the peril of their own political career, some members took it upon themselves to be the activists of the day.
They protested about a few issues, got branded as troublemakers, their character assassinated by the mainstream media and even got thrown into jail. But they have never given up on a better Singapore, and until today, are still members of SDP. They still are trying hard to get into parliament, so that they can more help shape Singapore into a better place.
Singapore now has a vibrant civil society environment and those members no longer need to be activists. They can now concentrate on being the politician they are and try to win votes, so that they can get into parliament and better help the poorer people of Singapore. As such, that is why there is this perceived change in SDP. They didn't change their ideals. They just get to channel their efforts to the preferred method.