I have a customer with a sportec 551D ecu and he is searching and cant find 1mb ecus only 551C with 512kb. I told him to not buy it because it won't start the car or won't behave well with sensors / tuning. Is this true or all these ECUs are interchangable no matter flash size?

In me7.5 this doesn't work because wideband vs narrowband - however all S4 are narrowband.

Can he downgrade to 512kb ecu, we can just immo off and tune it without issues?