Lot of users are doing a forced migration to Linux / Free Software. That is because that is the preinstaled OS on their computers at school/work . That's ok, but why a home user, non-techie would say I am going to install this free software instead of my propietary OS?. The new software should make then happier than the current one to migrate.
An example of migration because more features is hotmail --> gmail migration. People just got more features from the google mail than from hotmail, and some of them went for the new one. But this hasn't been a massive migration
Also a migration because of less features or you-have-to-pay now to get usual features can happen. For example gaydar is popular gay contact web service. But recently they are charging money for putting on-line nude pictures, so people started to move from it to another services offering the same features for free.
Now take a free (as beer) application as an example: IM. Most users around the world are using MSN, Yahoo or AOL. The users don't care about "systems" behind the application. They don't mind about MSNP9, jabber, nexus or whatever. If they are going to consider a moving, they would ask these questions:
- Can I talk with all my current friends
- Would I be able to do every thing I'm used to do?
- Would I get any new benefit from this migration?
- for the first time show an assistant importing previous user account/network
- Show his contacts as before, with all previous features, ofering full compatibility
- Offer the user the posibility of create a new more-featured account
- Invite users from the old accounts to get this new fancy client with more features
