I just bought a used SE end of January coming from a 6S plus, love the size but the battery did drain fast. I bought both a battery case and a replacement battery. Battery case was a bit bulky but I used it for a month just fine, still easier to carry than the plus sized phone. Had time this last weekend and changed the battery, wasn't hard to do at all.
Been using it just over a week and it's amazing how long it stays charged now with the same usage, well worth replacing but if the drainage is acceptable there's no hurry.