You make alot of sense except when it come to the extension..
I am an IT guy. I deal allot with file extensions.
here is an example of what im talking about
lets take ANY standard file format, open or not.
PDF = can be read by ANY version of adobe reader (some features will not be available, but you can always at least read the text)
DXF = No bones about it, this is a STANDARD when it comes to vector drawing.
DOC = a file created in word 2003 can be read in word 97, again there may be some formatting loss, but you can always read the words.
When microsoft abandon the FORMAT they abandon the EXTENTION as well
Word 2007, whos files can not be read by word 2003 or word 97, uses the DOCX extention.
I can not think of a single time that a file format was completely changed, that it did not get a new extension.
What you are doing does not conform to best practices. If newer software can tell if its 1 2 3 4 5 by the header and can read all 5 formats, and older software asumes that all .ild is format 123 based, then the Best Practice is to specify that a .ild is only to be used for format 123 and that .ildx (or whatever) can contain any of the formats and should be traslated based on its header.
and if your worried about all the format 45 .ild thats out there already, no problem, just always read format from the header..
This allows .ild to work in all old software no matter what because it will always be format 123, AND this transitions over to the new format cleanly and gives you even more seperation from the old format by auto saving changed art in the new format by default (making older formats selectable of course).
you are COMPLETELY changing the STANDARD for the FORMAT so you have got to change the extension.
unless you are trying to force people to buy NEW software...
ildx or ilda works for me