Yes, battery concern is such an insignificant issue with the F3HP that I regularly just leave it in the on position all the time.

Something of more concern though would be what you're used to metering wise. The FM3A (and I believe all FMs), like the FE-series, uses a match-stick indicator (needle style). The F3HP uses a backlit LCD (with a very lame backlit button that you'll never use). If you regularly use it in manual mode and rely on the meter indicator as a form of feedback loop, the meter display in the F3HP will just piss you off as it's an absolute measurement without a reference point. I mostly use it in a-prio mode when I can't be concerned with doing manual metering.