Sorry, missed this post last week.
If you move the Sky+ upstairs and run a second line you will be able to use it just as you can now. If you move it upstairs without an extra cable then you'll be able to rewind live TV but only record one channel and you'll get some weird messages if you try to use the EPG while recording a program.
We've got two Sky+ boxes (and a normal free sat one in the kitchen) running off an OctoLMB and it all works great. They only addition I'd like is the ability to share video from one box to another but I'm sure we'll get that eventually.