Am I missing something or did you not read the original post? That is the key point of the whole proposal. To be precise you can't force someone to migrate per say but by disallowing the spending of previous signature types you are in practice doing just that, forcing them to migrate or lose their coins.
Yes, apologies. What I meant is that some of us are not in favor of a "mandatory migration", but rather an optional one. To not freeze any coin, but leave them on the fate of their security.