There are 2 basic types of antifreeze, one is crap, and the cheapest it is alcohol based, the alcohol evaporates after a very short time leaving you with no protection.
The other sort is ethylene glycol, this is made from ethylene glycol diacetate, via saponification with sodium hydroxide or hydration of ethylene dioxide, very closely related to ethylene dichloride, which was used as a substitute for glycerol, ie nasty explosives, main constituent of DYNAMITE!!
Anyway it is ethylene glycol we are talking about, this is the best, there are some with a different makeup.
Ethylene glycol not only protects your engine coolant from freezing down to -51C, oddly adding more than 60% mix causes it to have a freezing point that starts to go up ie towards 0C, so no point in over 70%!!
It's other majoir advantage is it raises the boiling point of the coolant up to near 200C if in the right amount % wise, but it has fantastic heat transfer properties, and used in some air cooling and heating situations.
Anyway long and short by law all must be capable of mixing with each other, but due to some having different different additives for long life, best to stick to one, so you know exactly where you are at protection wise 30% will protect down to -15C, 50% down to -34C.
Just drain out what you've got and fill with ethylene glycol it is perfectly safe, and will readily mix with the small amounts left of other types in the system.
tabetha