what do you mean by changing streams from arts to science? If you were formerly an arts student for SPM, nope you can't change to science. I was an Arts student and my combi was PA, Econs, Literature in English and Math (the syllabus for arts). PA was compulsory, Econs is cool as long as you know your stuff, Literature of English wasn't offered in any Ipoh school so I had private tutoring twice a week (heard only less than 50 students take it each year throughout Malaysia) while it would be advantageous to have SPM Add math to take STPM math.
Other subjects commonly offered would be Accounts, Malay Language, Chinese Language, History, Pengajian Perniagaan...hehe and I am not too sure what else
Whether it is true that Arts students need better results compared to Science students to get into uni.....I am not sure. I had 2As and 2Bs and I got my first choice UM (in which I stayed only 2.5 days). Then again, you'd have to understand I chose a course that probably no one else wanted - Sastera (English). My other friends who had no As all got their first choices either in USM/UUM - for courses like Mass Comm and Management and Accounts. On the other hand, many of my friends in Science stream couldn't get their first choice.
But it's your call ultimately - where does your interests lie?