What is my best upgrade path?

I currently own two TivoHD units (with Comcast cable cards) that I've been paying monthly on for a LONG time (8+ years each) and I've replaced the hard drives and power supplies multiple times on each. I've probably paid more than double what it would have cost me to have bought the lifetime subscriptions in the first place, but that ship has sailed. I'm now in a position where I need to replace the hard drive on one system and maybe the other (as its become noisy), but I'm thinking I should...

What is my best upgrade path?

What is my best upgrade path?