How to convert from RSO to Cassini without errors?

People ask me whether QGIS can convert RSO to Cassini? If I tell them NO I can see expressions that signal 'Oh, most probably QGIS is inferior to proprietary GIS' If I tell them YES, they will ask 'How accurate is it?' so I avoid a direct answer and put them in a different perspective by asking them in the first place is it possible to convert RSO to Cassini or vice versa without errors? If the answer is YES, I say I too would like to learn how from them because simply I see both are of different projection. To exemplify, a RSO layer in Semenanjung Malaysia has Kertau, Pahang as its reference point so I tell them imagine shining a torchlight on a map of Semenanjung Malaysia to encompass the whole of it. The distance from the map to the bulb shall represent its projection scale. Now, if a Cassini map of Selangor is has its reference point somewhere in that state (don't know where) and another torchlight is made to shine encompassing that state, this distance from the map to the bulb will definitely be less than the earlier one for Semenanjung Malaysia. To a simple arithematician like me I see obviously will be error no matter whether done vice versa from Cassini to RSO but I am just informed by a senior cartographer friend that the error is 'very little and can be ignored' which I found this very interesting because I guess I think like a mathematician or accountant who want to close a balance sheet, every cent counts and on this matter, not like a cartographer. I was asked what is the difference between 1.0000000001 and 1.000000000009 or between 0.3333 and 0.333333? I would have answered it was relative to the user's requirement so who is to say which is more accurate because no matter what figure it is, it can be countered by someone who says mine is more accurate since it is more fine than yours like 'Anything-you-can-do-I-can-do-better' sort of thing which I have always encountered. Anyway, I would think a GIS software could  make up for error adjustment but the question is what rules were used and whether one agrees to that. Sometimes, people just because it comes from a particular GIS marque do not question it, do not care to think further and accept it as God's word. We can discuss this matter until the cows come home (if they do) but at this point in time, I have not finish with the subject coz I get to get some help, you must remember I am merely from the Arts stream and one who ended up muddling in GIS just knew sooner or later, in my field, I could not avoid it ... but until then keep surfing here, for update... if it ever gets done.