The only supposed benefit from changing to OAT is that it has a longer interval between renewal so a bit less work.
However the 2 definitely do not mix and unless you are trying to increase your service intervals I wouldn't bother.
If you do swap make absolutely certain that you have flushed out ALL the blue stuff because sometimes it can be trapped in some of the galleries or the heater core for example and then you will get some crystals forming which can in the extreme clog up parts of the cooling system.
Also, Silicone hoses prefer the old Blue Ethylene Glycol.
