Post edited to add the following clarification: many of my comments below relate to the full swathe of timetable changes resulting from the Hamburg-Hannover trackwork, not just the ICE route between the two big cities, as there are major flow-on effects. The point-to-point route is in many ways less affected, but it is simply too early to say.
There are indeed plans to redirect the Hamburg-Hannover ICEs rather than cancel them altogether. The most affected stretch of track is in fact between Lüneburg and Hannover - Lüneburg itself should still be accessible via Hamburg-Harburg, at least to an extent, but Lüneburg-Uelzen and beyond will be buses only.
(Unfortunately Hamburg-Bremen and Bremen-Hannover are also going to be affected by this - long-distance trains aren't going to be running on the latter line, though it shouldn't be closed completely.)
However, the possible [local] replacement timetable is proving a bit controversial (e.g. the local authorities in Lüneburg and Uelzen are not happy with the current plans, which don't include [m]any express buses), but it's not actually out yet. It's due 'early in the new year'. Definitely wait until the details are available before making any plans.
I'm afraid I'm a bit less optimistic than others about luggage space in rail replacement buses after some very bad experiences - it really depends on who's running / providing them. But if you get actual coaches for longer routes, it's fine - suitcases can go in proper storage below. There are bigger issues with more local replacements - if a regular bus turns up then it's a matter of squeezing on and hoping. They won't turn passengers with suitcases away, but if the bus is full, you're out of luck.
(Thankfully, planned track closures between major stations tend to work better than those due to short-term incidents.)
Anyway, wait for the revised timetable before deciding anything. Hamburg-Hannover is important enough that they'll have something functional set up. They've certainly had enough practice with the current Hamburg-Berlin trackwork!
(Background: I live on a nearby rail route with dodgy bridges in both directions, one of which has been out of action for a decade[!], the other on and off regularly and for the foreseeable future. Sometimes the alternatives are very smooth - sometimes they, er, aren't.)
