Let's say: It depends on where you are right now...
If you are already customer from Vodafone (either have a post-paid contract -or- a pre-paid plan), then you may simply switch between different plans (be aware that there may be issues or "waiting times" if you want to downgrade your contract).
If you are NOT customer from Vodafone but from another company (even if using the Vodafone network like Mobilkom Debiltel), you can transfer the number directly to Vodafone using the "Anbieterwechsel" function. Please be aware that your old contract will run to its end, even if you transfer your number to Vodafone immediately.
For transfering the number, you'll have to ask your old company to have a so called "Opt-In" set to your number - and in case of a pre-paid plan, you'll also have to assure that your account has enough debit in order to fullfill the number transfer (usually anything around 30 €).
