It is an MCB. There's an RCD on it also.

Why are you convinced that the main switch is an MCB? This would be
extremely rare. You mention an RCD as well. The normal situation would be to
have a main isolator switch (with a maximum current marked in amps) and an
RCD on the other side (also with a maximum current marked in amps). Neither
of these devices would trip when the current exceeded the maximum.

What are all the markings on the MCB you mention?

The main cutout fuse is the property of the electricity board. You're not
really supposed to touch it. Replacing a 50A fuse with a 63A fuse would be
potentially very dangerous, and probably illegal.

Is it normal for the fuse to have a lower rating than the MCB?

It is not normal to have a whole installation MCB at all.

Christian.