It will be highly unlikely that services in the VHF aviation band will be changed from analog to digital.
1. Whose standard would be used? Today, there are several digital standards being used for land mobile radio, ie Tetra (Europe and some other countries - public safety and commercial systems), Tetrapol (Europe and a few other countries - public safety only), APCO25 (USA - public safety) and Iden (Motorola's proprietry system used mainly in commercial applications, primarily in the US but in some other countries also). Just look at the bunfight going on over 3G cellular and whose technology should be adopted for the new global standard.
2. The logistics of a changeover would be a nightmare. Digital equipment would progressively need to be installed on aircraft (in tandem with existing analog radios) and likewise at airports. When this mammoth task is completed, the old analog equipment would ned to be decommissioned. And this would need to happen in every aircraft from a B747 to a glider in every airport from LHR right down to the smallest grass strip everywhere in the world, so as to ensure commonality between all aircraft and airports.