Quoted:
Li-Ion batteries are not a constant 3.7 Volts.
Surefire Lithium batteries are 3.0 Volts.
Rechargeable Lithium batteries are typically rated at 3.7 Volts, but when they are fully charged, they run at 4.2Volts and drop from there.
The only other Lithium rechargeable batteries are rated at 3.0Volts, but when they are fully charged, they run at 3.6Volts and drop from there.
RayOVac (Surefire) Lithium's (lithium–manganese dioxide) are not rechargeable and are 3.0V, constant, but some can be as high as 3.25V, this is before being but under load.
Good Li-Ion batteries come off the charger at 4.2V, but are a constant 3.7V. If you read the side of the battery and the spec sheet, it will tell you 3.7V.
RCR-123/16340's give short runtimes and if you use them in a head that is not rated that high, you will lose it.
Again, swapping out the CR-123's for RCR-123/16340's, is iffy. Better to mod the light, or to get a light that will work with them. Better still is getting a light that can work with 17670's or 18650's.