You specifically state you want use old firmware to remove restrictions to fly above 500m. Basically, admitting intent to break rules...
I usually update to latest firmware as soon as they came out. The only time I've seen DJI specifically locking their drone from taking off was in Sept last year (I only had the Spark then), they gave advance warning about that. That was something about battery firmware, which could shut power and cause the drone to fall out of the sky.
I'll leave it to others who are using older firmware to comment of DJI actually locks Mavic if old firmware is detected.