Found an old 2AA Maglite, can they be upgraded to LED?
Yes it can, but at a very dear cost.
The cheapie bi-pin drop-ins (eg. TerraLux) won't do much good as they don't yield good output and good regulation. Non-modders may be happy with that at first, but after using a generic light (eg. Fenix), most of them will just ditch the Mag.
On the other hand, custom sandwich modules require a of TLC, and will involve a lot of trial-and-error in the process of building them. Also adapting an LED system on an incandescent flashlight body is not as easy as expected. You can run a low-output module which is gentle to the batteries and yields a cool system, or you can go the other way round -- running a high-output module with less runtime and more importantly, heatsinking problem. For an average custom MagAA modder/user, striking a balnce between the two has been a problem.
To run a good custom MagAA LED system, you'll need a lot of things. Lens, optics, driver boards, custom heatsink (I sold some clones a while back), LEDs, clicky switches... all these will add up to a huge amount.
I too have been asked many times why I still stick to MagAAs. To be honest I've been carrying one as EDC for the last 15 years and I'm just being attached to this model. Even after I've lost one (with custom module and parts) I still insist to have a replacement (plus replacement module and parts). It definitely feels better than any modern style generic Chinese-brand light, especially given that such MagAA system are user-manageable and servicable (except the module, which must be maintained by serious modder/builder/etc.).