While we don’t know what technology is used, we can not give common hints for it.
Maybe you can find a proper back-fitting plan for a special loco in our examples.
1.) Engraved “D” on bottom side of gear is marking a 4-pin-gear.
Here are all 4 contacts for digitalization existent. Without “D” you have to rebuild.
2.) Old Interface for 6 pol. interface cable.
The decoder will be connect with the 4 gear cables and the 6 pol. cable.
3.) Direct decoder interface for LGB-55021 (or eMotion L)
The decoder will be connect with his pins headfirst in the PCB.
4.) OnBoard-Decoder for analogue and digital.
This loco can be driven without any changes analogue and digital.
5.) New 10-pol. DCC-Interface.
With the new 10-pol. interface cable you can connect a decoder very easily.