This is a real issue for many of us HTPC users.
Personally, I hope to hang onto my W8.1 with Media Center for quite a few years so I don't have to deal with it!
I tried NextPVR a year or two ago but was unable to get it working.
IIRC, mediaportal requires you subscribe to a TV listing service, does it not? I like with WMC that Guide listings are free!