The Park and Waterbodies Plan is part of the Master Plan 2003.
URA Master Plan 2003 website states that "For the first time, two new island-wide plans were introduced to guide the planning of greenery and identity for the Master Plan 2003 - the Parks and Waterbodies Plan and the Identity Plan. The two plans presented ideas on how to enhance greenery and identity as to improve the living environment. They were drawn up arising from earlier public feedback from the Concept Plan 2001 that the public valued identity and greenery."
According to the URA website on the planning strategies of Master Plan 2003:
Pulau Ubin was spotted on the model map of Singapore at URA
It seems like there are plans to keep "a large part of" Ubin as it is, for now. Hopefully it would not be taken for granted that a standstill would suddenly occur on the island. It still remains to be seen how much of changes might occur on the island.
Even so, URA's recognition and attempt at preserving Ubin is absolutely commendable. We can only hope that this could continue for a long time to come.