The relevant provisions of the Act which deal with s 106 modifications in relation to affordable housing came into force on Royal Assent. CLG has also published some guidance on these new provisions - in particular in relation to the viability evidence required to support an application for review.
No particular methodology for viability assessment is prescribed. An "open book" approach is strongly preferred (although not essential) and the starting point will be to review the viability assessment which informed the original grant of planning permission (assuming there was such an assessment). The assessment submitted to justify a review should follow the same methodology as was used in the original application - with any changes properly justified and explained. All other obligations (ie non affordable housing obligations) will remain unchanged - so the updated viability assessment must assume them and focus only on affordable housing.
Note that where an application to review goes to appeal, the Inspector can only allow a reduced affordable housing requirement for a 3 year period. After that, the requirement reverts - save for parts of the development already commenced. The guidance suggests that local planning authorities may want to time limit any modifications it is minded to make.