I know this has been a hot point of debate and I'll admit I didn't slog through all of it so forgive me if someone has already argued this point with you.

I think when you take the oaths, they cut whatever remaining lifespan you would have had roughly in half. The WT Aes Sedai don't live much past 300, but one and all they swear the oaths very early in life. So their projected remaining life was approx 600 years, they get only 300. If Mesanna has lived about 300 years, and then took oaths, I think they would take her remaining 300-400 years and cut them in half.

She would not appear to age on the spot, or die, or appear ageless. (note: I think agelessness comes from time spent under the Oaths, not age + oaths) So Mesaana would appear exactly the same, but her projected lifespan would be only 150 or so more years. After 20 years she would look ageless just like an Aes Sedai. But none of that last bit would happen if she removed them quickly.

Point: by this reasoning, Siuan won't suddenly look like herself again when she retakes the oaths. She will gain the agelessness in about 20 more years, and since her clock was reset, she should live a bit longer than the other Aes Sedai.

I think this theory is supported by a similar RJ commentary:

RJ's blog 20 January 2006 "IT'S BEEN A WHILE"

- Regarding Sharina, and other women who learn to channel at age, she will indeed grow younger in appearance. No, she will not achieve an Aes Sedai face without the Oath Rod, but where she has previously looked, say, sixty, she will look perhaps thirty-five, with accompanying changes in hair color. Think of it as analogous to slowing, which older women also do.

So the power makes women age much more slowly. If cut from the power, you go back to being young. If granted the power in old age, you become young again. I don't think this evidence points to the three oaths aging someone on the spot.