Please note that since 1.9 will be another update on the mapping system, there will be no new map when 1.8 comes out. Instead you will have the chance to regenerate your lot or to move to another lot that is regenerated with the new mapping system – on the same world. If you will be able to switch to a new lot or regenerate the current lot will have to be evaluated. Either way, you can keep your current lot as it is if you want. And, please note that if your lot is regenerated or if you move to a different lot, the current lot in question that you own will have to be cleared by you of everything that you want to keep. Either you move the stuff temporarily into a shop (only for a day or so), or you find an alternative space for it, be it in a different lot you own or on a friends lot.
So what about 1.9? Ideally, we do not start a new map either. However if it’s heavy biome AND landscape changes (height & location of oceans etc), we will start a new map then to have a clean start. It will NOT be a forced update for anybody, people can choose if they want to move to the new map or not. If it is ONLY biome but no height/water changes, we will not start a new map. So as you can see, it really depends what they are doing with it. It is be hard to tell.