The LAPL does not have an expiry date It relies on rolling validity and requires a set amount of experience in the past 12 months on any day that you fly.
Changing from a C172 to a PA28 does not require Differences training, only Familiarisation training that need not involve an instructor. Differences covers VP props, turbochargers, presurisation, Nose/Tail wheel and retractable undercarriage. For SEP and TMG aircraft if you have already flown them it does not need to be repeated.
A LAPL will only cover SEP and TMG class aircraft there are no Types to transfer.
I believe there is an AltMOC that exempts you from the 10 hours which is applicable on initial issue rather than when transfering to a LAPL.
I believe that EASA intend to include LAPL privileges into the PPL so there may not be any need to change licences if you can afford to wait a while. You will just be able to drop to a LAPL medical.