u can downgrade, walter made an fixed-sbfi think rooting is possible for the leaked GB roms on Defy (not Defy+) but downgrade is not possible (but who would wanna downgrade?? right)
As for Defy+, its all just speculations at the moment, the bootloader may be changed, maybe new locks, or maybe its under google now, so unlocked??
defy+ ist just defy with red-lense and slighty bigger accu, flashing defy+ leaked ginger will make a mb526 out of your 525.
already did that and flashed back to froyo, because i had to be repaired, no problem at all
cpu is the same, the hole thing is the same, for gingerbread-update u have to buy a new phone