I went through this same decision process with my trusty MagLites a few years back. As painful as it was to see 4No expensive torches go to waste, in the end it was MUCH better to just buy a new good quality LED torch as they are almost the same cost, much smaller/lighter (so use cheaper batteries) and are built for purpose (rather than a 'conversion').
I bought Fenix torches which all run on rechargeable AA batteries, but LEDLenser make much the same thing, are a bit cheaper and often come up at very good prices on Amazon.
My one cell AA torch is about 3x as powerful as my old 4 D cell maglite and weighs virtually nothing by comparison. My two cell AA torch is brighter than a car headlight on full beam.
I use Sanyo Eneloop batteries and a good quality charger. It was all a bit of an investment, but we use our torches on a daily basis and if they died tomorrow I would buy exactly the same again as replacements. Very highly recommended and I don't miss my MagLites at all now.