In my experience if changing from DOT4 type to silicon type without a complete strip, clean, seal change some of the old fluid remains in the system and leaches back out over time which means that repeated fluid renewal is required over many months/years to get rid of all of it and even if it is OK to do that it doesn't seem a good idea.
Silicon is great if you have a leaky cylinder as it doesn't dissolve the paint on your bulkhead and it probably lasts longer than DOT4 but be warned, it does still need changing every few years. When new it usually has a strong deep purple (smoke on the water?) colour but this fades over time and ends up a pale coffee colour if left in the system for too long, so renew when it becomes noticeably paler.
I would also advise those 'press on' drivers not to use it as, if it overheats (I've seen this several times on track days and Alpine passes) it turns to jelly and, unlike DOT4 type fluids, doesn't return to a useable liquid state, albeit not as good as before boiling, which requires the whole system to be purged and refilled with fresh fluid which isn't easy by the side of the road halfway up an Alp.
In short, if yours is predominantly a show/light use car then you may be better off with silicon fluid but if not and if you are likely to get the brakes pretty hot, use Super DOT4, DOT4 Plus or even RBF (race stuff) with a higher b.p. and change it at least every 2 years.