Yes, to change the receiver you will have to break vacuum when changing it. Haste is important as oxidation does occur however a cow really offers very little to this at a small scale in comparison to what it takes away. My point is that storing the terpenes in a receiver on a cow might prevent some oxidation but the fact the volitiles are being stored at all inside the vacuum deteacts from it. Even storage inside a cold finger will degrade vacuum because the stuff is always evaporating even if slowly.
I routinely must break vacuum on my rig. So I take advantage of the situation by allowing the pump to gulp down copious amounts of air while I attend to harvesting the coldfinger that my sublimator uses. Flushing the system with fresh air from having to break vacuum has an offsetting benefit insofar as it can restore vacuum a bit by flushing volatiles out of the oil. Plus doing this on the entire vacuum path rather than an internal gas ballast that most vacuum pumps have helps to move along volatiles that are heading down your vacuum hoses and such. After I started doing this on average 60 seconds or so each time I needed to break vacuum my oil changes to restore vacuum were cut in half. So there are almost always more than one facet to consider when making a chouce like this. I have found the oxidation to be inconsequential but I also waste no time when hot extract is exposed.