Why should (s)he give up the ATPL?
Just add the Class Rating SEP to the holding license and that’s it.

The pilot can only hold one aeroplane licence, so will need to add an SEP Class Rating to his/her pilot licence. His/her medical certificate should include both class 2 and LAPL privileges, so if his/her medical validity drops down to LAPL level only, he/she may fly within the VFR/2tonne/4PoB limits, but only if he/she has maintained SEP Class Rating validity, NOT the 'rolling validity' of the LAPL.

A higher level pilot's licence (e.g. ATPL) should give the holder all the privileges of the lower level licences (e.g. CPL includes privileges of PPL and ATPL includes privileges of CPL and PPL) with the exception of MPL.

A medical certificate is also required to exercise the privileges of a pilot's licence. For example, to make use of all privileges of a CPL or ATPL, a pilot needs to have a valid class I medical certificate. They could use the pilot's licence with a class II medical certificate as well, but then they could not use all privileges that the licence provides.

But the pilot's licence and medical alone do not give the right to fly an aircraft - for that a type or class rating is required. So in order to fly a SEP class rating aeroplane, the ATPL holder needs to get a SEP(land) or SEP(sea) class rating.
This can be done by completing a class rating course at a DTO or an ATO. Another option, in case the pilot already once held a SEP class rating, is to renew the rating, but if the rating expired a very long time ago, then this would be almost or exactly the same as completing a class rating course.

There is no requirement for an aeroplane class rating if the licence holder exercises LAPL privileges and the rolling validity may be used. This was the intention of EASA as clearly expressed in its rejection of a comment proposed by UK CAA. EASA CRD 2014-29(A), published 29 June 2017, comment no. 655, pp 106-107. https://www.easa.europa.eu/en/document-library/comment-response-documen…