Old rule: "The race number bib – this must be worn on the front & the logos of the race sponsors must be visible."
New rule: "The race number bib – must be visible but it may be positioned elsewhere than front. Logos and race sponsors must be visible"
Hint: By utilizing elastic band as e.g. triathlonist do it is easy to carry number bib always visible despite the need to change clothes. With elastic band you also slip of puncturing your garment you could get from using safety pins.