Conversions in General

Land Rovers have and still do lend themselves greatly to modifications in all areas because they have a very basic structure, with very basic components. All in all, throughout history, I don't believe that any single part of a Land Rover has been left original.

Before considering altering or replacing something, it is important to establish exactly what you want to achieve and why you want it. You must consider all available options and remember to calculate in unseen costs, you'd be amazed to see how the cost of a conversion will add up with the little things.

Land Rover engines can and often do last very long but more often than not the vehicles are required to perform better than originally intended. Land Rover's original design is very good and robust if kept as a unit, but, as soon as any upgrade takes place, the rest of the unit must be assessed for compatibility. For example a significantly larger engine conversion will be detrimental to the gearbox and braking system.
It must also be kept in mind that the existing engine can ALWAYS be upgraded to increase performance and fuel mileage, It is often a much better option to keep the existing engine and tweak it.

Gearbox Conversions usually happen with engine conversions and are rarely done on their own. The exceptions going from manual to auto or visa versa. 

Series vehicles usually keep their gearboxes as they work well and are relatively cheap and easy to repair compared to the newer boxes.
Sometimes of the coil sprung vehicles gearboxes die and are replaced with newer models for better gearing and reliability.
 

Diff Conversions exist and are used either for gearing upgrades after engine conversions or for performance upgrades like diff lockers etc.

Suspension upgrades are becoming popular in light of camping holidays and long distance travel where a more stable vehicle is desired when fully loaded.

Brakes are sometimes upgraded like drum to disk conversions on leaf sprung vehicles, There are other bits one can fiddle with too like cooling, interior, body etc.

 Unless researched properly, what started out as a very innocent upgrade turns into a very costly and dangerous mess.