Possibly to apply for ancestry visa while still on student visa

Hi everyone, pretty much as the title states, im in the 3rd year of a 4 year student visa (1 more year of uni left), and I'm considering my post graduation visa options. I have a british parent and grandparent and am also a commonwealth citizen(i think, I'm Singaporean. I dont want to get british citzenship though im eligible because id have to revoke my singaporean). I understand that one cannot "switch" into this visa while still in the uk. What constitutes "switching", however? Would I have to wait till my course is completed and return home to apply? Or wait till my current visa expires? Or can i apply even now? I am currently abroad in singapore for 5 weeks, which is more than the 3 week processing time. I have a part time job in the UK on top of my studies, and have savings that i can use to support myself. Would I be able to apply for that visa now and secure the next 5 years, or must I wait? Has anyone else applied while on a student visa? Thanks so much!!