I've read somewhere else on these druid forums (resto) that one should swap out 4p T12 directly over to 4p T13 when you have them.
And not go with "both", meaning 2p T12 + 2p T13 is a loss in healing. (Not sure how much/little.)
And one should change it of equal quality too. If you have 4p T12 with 2 of them HC, one should only swap to 4pT13 when you also have 2 of them in HC for example.
The 4p T12 is really good, and the 4p T13 is meh, so you need to cover the "loss" of 4p T12 bonus with higher ilvl 4p T13.