Personally I don't really see how the conversion process could cause trouble, at least for Vanilla worlds that is. All they have to do is a fancy "search and replace" that looks up the blocks by ID and then replaces them with a name in the save file. Assuming that's all they change.
For mods that will cause issues though, unless they modify Mojang's converter to also replace their blocks properly. It's not impossible, but the main question is if modders are willing to invest time in a converter when the issue is easily solved by starting a new world...
Personally I'd rather have modders focus on updating their mods at that time than bother to save everyone's worlds. I mean, making 1 converter for all different mods will be a huge challenge and take lots of time. If I get the option to play with mods a week after the release of the API, but lose my world. Or wait for 2 months and not lose my world, then I will just drop my world and make a new one.
Chances are there will be changes to world gen and stuff by that time anyway.
For mods that will cause issues though, unless they modify Mojang's converter to also replace their blocks properly. It's not impossible, but the main question is if modders are willing to invest time in a converter when the issue is easily solved by starting a new world...
Personally I'd rather have modders focus on updating their mods at that time than bother to save everyone's worlds. I mean, making 1 converter for all different mods will be a huge challenge and take lots of time. If I get the option to play with mods a week after the release of the API, but lose my world. Or wait for 2 months and not lose my world, then I will just drop my world and make a new one.
Chances are there will be changes to world gen and stuff by that time anyway.