Replacing NI-CAD Batteries with NIMH or LION

My thought would be that you should follow the manufacturer's instructions about discharge cycle AND recharge cycle of whichever technology you choose.

Follow the instructions to the letter!

Not much else.

Well, ensuring the voltage is the same for one. Does the laser beacon have its own built-in battery charger or separate battery charger that the beacon plugs into, if so it is unlikely that the battery charger is designed for NiMH or Lion battery charging (ie- you will likely need to buy a new battery charger if you change battery chemistry).

Again, carefully check your beacon documentation.

"Well, ensuring the voltage is the same for one" Duh. I just assume that the OP knows about battery types and voltages. I sometimes overlook the obvious, but your advice is solid.

As you say, read the instructions.

I have successfully replaced NiCad packs with NiMH in cordless phones. For maximum lifetime and performance, the NiMH really do require specific charging curves, but I haven't blown up any (yet).

Do NOT use LiIon batteries unless the charger specifically supports them. Lithium-Ion cells are notorious for violently self-destructing if over-charged. 

For practical purposes, wherever you use a NiCd battery, you can replace it with a NiMH battery. They come in the same sizes, have the same voltage, have greater capacities, do not suffer from the memory defect problem, can deliver greater currents, and can be charged using the same chargers. Li-ion batteries, on the other hand, have a different shape (cuboidal rather than cylindrical cells), a different cell voltage, and MUST be charged by a dedicated charger. The particular bonuses of Li cells (high capacity, high current capability and low weight) are unlikely to be relevant to your application.

As has already been stated, there are strong similarities between NiCd and NiMH cell systems. Voltages are similar, chemistries are somewhat similar, and physical configurations (ie. AA or C cells) are often interchangeable. LiON cells are a whole different breed of cat (apologies to Del). I suggest the following as a fair source for needed background info on batteries:

http://www.buchmann.ca

My final recommendation: Charging circuits for LiON cells are very sophisticated systems requiring precise voltage measurement and temperature compensation. They incorporate overcharge- and undercharge-protection to avoid plating of metallic lithium (a metal more chemically active than phosphorus or sodium) within the cell enclosure. For safety, and for cost reduction, leave LiON applications to the experts.

Check out this site for lots of battery information. I think Battery University answers all your questions and gives a link to Buchmann's book on line. Its a good reference site.