I was flying once where I wanted a seat swap but the most likely pax hadn't boarded yet. (Elderly, Alzheimer patient MIL much easier to manage with two of us vs one of us nearby, but we would have survived so no need to get FAs into it.) I left a note on each seat asking, saying which seat I could swap, and that no action or verbal denial was necessary if it didn't make sense for them. One read the note, smiled, took their seat. Perfectly acceptable. One caught my eye and graciously offered to swap. I didn't feel comfortable assuming or putting them on the spot, especially as it was a long haul.