I not sure I agree with this. Scarifying (or road planing) is mainly used for the purpose of lowering the existing level in order to create falls for drainage or to ensure that the finished level with the new macadam ties in with existing features such as kerbs etc.
Even when new roads are laid, macadam is normally laid in layers without scarifying in between. Often there is quite some time between the layers being laid (for example almost every housing estate in the country).
The only special treatment of the lower layer required prior to laying the upper layer is that it is cleaned and in some cases sparyed with a 'tack coat' to aid adhesion.
There is one circumstance in which the lower existing layer would need to be removed. If it is structurally inadequate. In other words, if the existing is ravelling, then just overlay it. If it is settling (and maybe ravelling as a result) then dig it out, recompact the granular sub-base material adding more granular material as necessary and start again.