The Sea-Wolf class was replaced after 30-33 years of extended sterling service. If you go along those lines, the Victory class will be replaced before the end of the next decade with orders starting early in the first half of next decade. The were ordered in 1983 and the first ship commissioned in 1990. ST Marine naval yards have got something to do apart from building the new Landing Platform Dock for the Thai navy.I'm not sure if the Victory and Fearless Classes of vessels will be replaced yet, but there is at least a need to have plans to upgrade them, if they are to remain in service.
It may well be an indigenous solution and it may have something to do with ABM defence;something Singapore is vulnerable and currently looking into. ST showcased this new stealthly frigate design not too long ago;
http://img519.imageshack.us/img519/4421/4e7a0aa7c8837477b5994e9xb7.jpg Credit: Local Mil-nut forum
However, priority would still be new submarines to replace the Challangers which are even older(but extensively upgraded); possibly the A46 submarines. They could opt to build them here with suitable technology transfers or simply have them built in Sweden.
With regards to the Asters on board the Formidables, the RSN has never stated they were carrying Aster 15s only. Foreign observers did though they had few information to rely on then. Officially, RSN simply refers to them as Aster surface to air missiles.