The difference between the files of 3.5 and 4.0 aren't much more than code differences, so why not make a tool which converts DMIs & DMEs from 3.5 to 4.0?

This would help bring more BYOND Players & Developers to use 4.0. 4.0 is already at quite a Stable version, I don't think it would be a problem to discontinue the use of 3.5.
Icon files are already automatically converted, and that's the only real file format difference between 3.5 and 4.0. You can already open a 3.5 project in 4.0's Dream Maker and compile it fine.
In response to Nadrew
:o I forgot, I was thinking 4.0 - 3.5
In response to Magdreamer
Not gonna happen, the formats are majorly different and it would be working backwards to convert them back.