What processors are you running on? The current software release (4.5) is the last one to support the Motorola processors (CP3 and CP4), so when software release 5.0 comes out, if you're on Motorola processors now, the software upgrade will require a hardware upgrade to the Pentium 4 processor, and if you have an Option 81C, you'll also have to upgrade to Fiber Network Fabric. This will transform your "big" upgrade into a "huge" upgrade.
Changing your RPEs to Carrier Remotes now might enable you to phase in the rest of the upgrades (RPEs now, software in the fall, then processors sometime later). Release 5.0 is currently scheduled for release in the 4th quarter of 2006. When it goes GA, it forces the rest of the upgrades. Look for pricing incentives later this year.