Hi Sophie and welcome to the forum. On the assumption that you are also new to the site this link should help you find your way around.
Welcome to Benefits and Work People on indefinite DLA will be 'invited' to claim PIP some time after October of this year. You will have to request the forms from PIp, fill them in and submit them withina specific time-frame.As long as you do this, your DLA claim will continue until 4 weeks after the PIP decision is made.
If ATOS/Capita think they have enough information to make a recommendation to DWP they will do so, otherwise you will have a face2face assessment. Capita tend to carry these out in the claimant's home whereas ATOS usually invite the claimant to an assessment centre.
The foundation of the welfare reforms is that everyone on state benefits will be subject to periodic reassessments, it is too early for us to know what length of awards will be most common.