Nextel and Boost use similar phones. For example the i880 and the i885 are the same phone but the i880 is for boost while th i885 is for Nextel.

They use the same firmware (OS) only difference is the i880 is cutomized for boost while the i885 is customizedfor Nextel.

So can you use your i880 on Nextel or your i885 on boost? Yes but you may run into some issues.

How do you fix these issues? Well you update the firmware with th OEM firmware available from Motorola.

Where do you find the firmware?

Here
http://idenphones.motorola.com/iden/support/software/html/firmware_utility.html

What do you need?
A computer and a USB Data cable for the i880 which can be found for as little as 7 dollars
http://cgi.ebay.com/NEXTEL-i875-i880-USB-DATA-CABLE-RINGTONE-CD-FREE-i-875_W0QQitemZ230125528267QQcmdZViewItem

What do you have to do?
Download the software for your phone and run it on our PC. Make sure your PC meets the minimum requirements and you have your phone connected to it.

WARNING DOING THIS WRONG CAN RUIN YOUR PHONE, ALSO EVERYTHING WILL BE ERASED