5.5 will mean a new game and an updated CF, in line with the latest Unity "improvements". Unity 5.5, however, is out and about already and PP usually follow one or two updates later, since they have to catch up belatedly.
Unity 5.5 will mean procedural grass aka TE (turf effects) and a few more improvements, mainly in lighting and in terrain tesselation. It's going to be great, they say. If the bliss should happen, you should get better and nicer images at the same or even less performance cost.
It should also mean the advent of JNPG2 with new avatars, new swing mo-cap, with career mode, console versions, an improved user interface etc and many, many, many good things more. This being a major overhaul of the entire project and the primary focus of the devs atm, that also means that the game and CF we now have will be left mostly unsupported and un-upgraded, until the switch to 5.5 will be performed.
I'm not sure you will have to upgrade, Ola - of course we all are supposed to upgrade our stuff all of the time because by the time we buy new computers they are already obsolete and overshadowed by the next amazing innovations coming down the pipeline. So it's up to you if you want to go for a panic buy or just wait and see. My personal recommendation would be to think twice about the former and opt for the latter.