OP, you are new modeler with an old plane, engine and electronics. The last thing you need to do is dick around with different battery technologies that have different voltage outputs. Yes, replace all your batteries but use NiMH batteries instead of LIFE or lithium. NiMH are direct replacements for NiCds and are readily available. They have also greatly increased their amp-hour output. I have over a dozen 2300mAh AA batteries and in total cost less than $50.
IF IT WORKS, DON'T "FIX" IT.