Hi queenmalka
The migration criteria from DLA to PIP have nothing to do with whether people have indefinite awards. It is about the age group they fall into. Anyone born on or before 8th April 1948 will stay on DLA. As your relative was born later, there is still a chance they will be asked to migrate at some point. Now, the migration process has been paused and is not expected to start for the majority of those still on DLA until 2028, but there haven't been any recent announcements about it.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
