Except for Autocad most CAD programs do not save backward compatible versions. AC has a workaround which is to go back one version at a time. From 16 to 15 to 14. This procedure has the drawback that you might loose information along the way. For example 15 does not have the Morph tool so any Morphs that are not saved as objects will disappear when you save as AC15. ALso if I remember correctly AC14 does not have shells so those too might disappear along the way.
If you need AC15 you can request it from your reseller it will work with your current key.
You can also try an IFC export, which will let you go from 16 directly to 14 but there can be surprises along the way.
Eduardo Rolón AIA NCARB
AC29 US/INT -> AC08Mac Studio M4 Max 64GB ram, OS X 10.XX latest
another Moderator