Most people with 1 cell Fenix lights report a loss of low mode with a 3.7V cell, and report a brighter than normal low mode on a 3.0V RCR123 cell. Some people report burning out the electronics or the LED. (this is possible in some revisions of the 1 cell lights over the years, especially if it has a relatively low Vf LED).
I'm not a big fan of the 3.0V rechargeable options. Either you sacrifice energy density by going with LiFePO4, or sacrifice by wasted space and energy converted to heat with a voltage-bucked cell. both options have a lot of down-sides.
My recomendation when this question comes up (I'd estimate I've answered this question around 57 times in the last few years), is to sell the 1 cell Fenix lights as buy a competing model from another brand that offers 3.7V RCR123 compatibility. In the long term, this will simplify things. If you buy the charger and cells to make the Fenix rechargeable now, then in the future wind up buying different cells and different chargers for other lights, then there are possible dangers that can be introduced that are unnecessary. (Many of the chargers designed for use with the voltage-bucked 3.0V cells can cause a regular 3.7V cell to explode if accidentally put on the "3.0V" charger).
-Eric