When a consumer unit actually needs replacing
A plastic enclosure, an unfamiliar brand, or simply being old are none of them reasons on their own to change a board. Since January 2016 a domestic consumer unit has needed a non-combustible enclosure — in practice steel — which is why newer boards look different, but an older board that tests fine is not dangerous by virtue of its age.
The one worth spelling out: a circuit that keeps tripping is a fault on that circuit, not a faulty board. Replacing the board without finding the fault means paying for the board and then paying again to find what was tripping it. Fault finding first, board second, is almost always the cheaper order.
There are real reasons to upgrade — no RCD protection where it is now required, damaged or overheating terminations, or a board that cannot take the circuits the house now has. When we do change one, we talk through RCBO-per-circuit against a split-load arrangement, and surge protection gets assessed and recorded rather than fitted by default. A consumer unit change is notifiable work in Wales, so it comes with an Electrical Installation Certificate and a building regulations compliance certificate within 30 days, and you never need to contact Newport City Council yourself.