Hi Mohamed,
Most of the Summit X440 switches have a minimum software version requirement of 15.1.2. The X250 supports a maximum version of 15.3.x (15.4+ will not work).
Since you won't be able to downgrade the X440, I would recommend that you upgrade both switches to the latest version of EXOS 15.3 (currently v15.3.4.6-patch1-10).
There is a reference guide you can use to determine what versions work on each model of switch.
ExtremeXOS Hardware/Software Compatibility
As long as you have valid support entitlement to download software, you can get it from our support website:
http://support.extremenetworks.com/
Follow the link titled "Downloads for BlackDiamond Series, Summit Series, E4G, ExtremeXOS and EOS."
You'll need to login to get the EXOS image download. The "summitX" file you get for the X440 will work on both switches.
Let us know if you have trouble!
-Drew