If you move, get an install of TiVo and TiVo2 comes out, no way VM are going to tell you you cant upgrade- and you have the agreed discount in pace..
As to what the terms are, usually as VM "rent" the equipment to you as part of your contract, any additional charging for an upgrade usually gets marketed as a one off activation charge. These are usually negotiable, and fall off sharply. I THINK Tivo used to be 50 quid? now its a tenner... so worst case thing something in that ball park
As for timing, no one is sure. Internet pundits are saying October, personally I think that's a little early, Q1 2017 is my guess. But that IS a guess and VM have a habit of surprising us...
There's not a lot in 4k ATM and my advice for anyone thinking of buying a 4k TV is to wait, HDR is now on most new 4k sets but only LG OLED's have Dolby Vision and HDR meta data is apparently coming soon, also the BBC and several other broadcasters are making new transmition standards for 4k and these are not finalised as of yet so to get built in freeview at 4k (if you require it) it's best to wait