Swapping US phone with Docomo

I bought an unlocked iPhone 6 straight from Apple in the US and it works in Japan. I have a SIM card and contract from Docomo.

I am visiting home in December and would like to swap phones with a sibling of mine. He wants my 6 and I want his 6 Plus. It is also an unlocked iPhone 6 Plus straight from Apple and thus should work in Japan.

My question is, if I take his phone and put my SIM card in it when I'm back in Japan, will my service with Docomo go back to working the same as it did before I left with my regualt 6?

maybe, you can use the 6 plus.
OK if the size of sim card is the same.

Should work fine once you install your backup onto it (so make sure to backup before you do the swap). Otherwise you may have to update apn settings to get it to connect to the network.