N900 have recieved new base (NF3) and which have new kernel, Once you use this new base, your EFS partition will get modified and you won't able to downgrade device. in fact you can downgrade but you will lose IMEI.
To keep possibility to downgrade you must have backup your EFS BEFORE flashing new base
You have to use new modem on older base.
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?