Well, you can't go backwards and keep those configurations intact. Can you get the 2015 files as parasolids or STEPs?
Thanks. Is there a way to create STEP files from within SolidWorks 2015? So what I understand from you is by importing STEP files into SolidWorks 2014, the assembly integrity is maintained and also I won't lose the configurations.
In 2015 just do a save as and select the appropriate extension for whatever conversion you want, in this case .step.
You will lose all configurations when you open in 2014 as a step conversion. I'm sorry, I didn't mean to come across like that. Depending on complexity of each component, you could do a "feature recognition" but that takes time.
Exporting to STL or IGES will give you a relatively 'dumb' file. It will not keep the configurations of your 2015 assembly. You will need to re-create configurations, mates etc.
You'd have the export (parasolid) all configs out and re-import them (x_t) into your 2014 file and set each body (delete other bodies) per config.
If it's a lot of data/configs.. there are some macros which may help the process..
btw, Parasolids should be your first choice, SOLIDWORKS uses a Parasolids kernel...
Second option, STEP.. and lastly SAT (Acis) or IGES. (imho, forget the idea of using IGES or STL)
and,.. Parasolid version per release
2012 = 24
2013 = 25
2014 = 26
2015 = 27
2016 = 28
MACRO - Save As Parasolid with Revision after Part Number
Macro to Export Multiple Configurations
Exporting Configuration Data