It is only compliant because it is an existing situation.
As soon as you demolish one of those stands all the current regulations regarding emergency evacuation, access for emergency & service vehicles, disabled access gathering areas for up to 1000 people so they arent queuing on public roads, structural design, leg room, toilet & catering facilities etc will kick in for whatever is built in its place.
Speak to Blootoon red at what is involved for a football stadium.
If you live in an old house and you want to make some changes to the layout then everything you do will have to comply with the current building regulations. What you don't touch can remain the same but that new extension will need to be energy efficient, have doors of a certain width, windows of a certain design and size, any bathrooms or kitchens will need to comply with the accessibility regulations.