Can’t exactly see a fair process being rolled out here….this Treasury paper strongly implies that the 20% cut in DLA expenditure that the government expects to achieve by phasing DLA out was drawn from results of the existing, and flawed, work capability process.

From Treasury 2010 budget costings documents (see page 36)

Reforming disability living allowance – 

Measure description
This measure will introduce an objective medical assessment and revised eligibility criteria for both new and existing working-age claims for Disability Living Allowance, to be rolled out from 2013/14. The assessment will follow a similar process to the Work Capability Assessment (WCA) used for claims to Employment and Support Allowance, with a points based system to assess eligibility to the different rates of the benefit.

The cost base
This costing uses the same caseload and expenditure projections used in the main social security forecast.

Costing
Drawing on the evidence of the impact of the WCA, the central assumption for this policy is that it will result in a 20 per cent reduction in caseload and expenditure once fully rolled out. It is assumed that existing claimants would be reassessed over three years, with 25 per cent of the caseload reassessed in the first year, 75 per cent by the end of the second year and 100 per cent by the end of the third year.

Touchstone blog from 2010 on that 20% cuts figure in DLA here.