No need to downgrade to 2.4, as YaBB 2.5 and 2.4 are pretty much the same.
With you YaBB 2.5AE:
Put in Maintenence Mode
Run all the maintenence functions from Admin Panel
Use the YaBB 2.4 to SMF1.1.14 scripts
Once migrated to SMF1.1.14, upgrade this to SMF2.0
Fix you boards and permissions (these will be screwed up badly), and any membergroup errors
Upgrade to SMF2.0.1
I migrated my 1.1m post, 100,000 thread, 11,000 member YaBB 2.5AE to SMF2.0.1 this way.