I have upgraded my 2Tb Seagate TC to a 4Tb Green WD.
Works well, and not that tricky.
Hardpart was twofold:
1 Getting the thing apart is not that easy.......get yourself some non scratching plastic spludgers that are STRONG. I broke off the end of one of mine getting the bottom off.
2 The foam in the bottom of the TC is designed to take SEAGATE drives. The metal folding is slightly different in the SEAGATE. To get the WD to fit snugly I had to take a sharp knife (I used a scalpel) and slice off the bits of foam that are sticking in the way of the bottom of the WD drive. Once I did that it fitted in the case as a factory fit.
I am actually thinking of buying a 6Tb drive and upgrade it again.
Cheers to all.
PS the price from Apple of the 3Tb drive is crazy. Get yourself a refurbed 2Tb and upgrade it to 4 or 6 Tb.