It's happened to all of us - you find the right person who is indispensable to the covenant, you buy them a longevity ritual and then 5 years later, blam. They change.
But how do they change?
Flaws such as Offensive to Animals or Air of Magic are candidates, but what other flaws could you suggest for warping due to Longevity rituals?
New Minor supernatural flaws
You gain a greater appetite, but your body remains the same general shape. You must eat more often than normal, eating as much food as 2 people would.
You suffer food deprivation after 12 hours rather than 3 days.
You gain a greater thirst than normal. You must drink regularly and often. You suffer deprivation rolls after 4 hours rather than 24.
You are visibly different from normal humans. You may have blue skin, green hair or webbed fingers. Whatever the difference everyone who meets you can tell that you are not a normal human.
When you go to sleep, after a diameter of sleep you enter a magical sleep with sun duration. You cannot be woken by mundane means until the duration expires. This is a supernatural effect and does not cause warping. If magical attempts are made to wake you they need a penetration of 25 to wake you or disrupt the magic.
Your skin has changed to become like stone, when you stand still you can be mistaken for a statue. You have a -3 to all activities using dexterity or quickness. You gain +8 soak but you lose all sensation in your skin. You weigh 4 times as much as normal and walking any distance leaves you tired.
Smell of breath
Something went wrong and maybe died inside you. At least everyone can smell it. Foul tasting mouth tonic (or garlic) might help a bit for a little time. During a direct face to face conversation your partner will keep forgetting what you say and will focus on the smell of death.
Aging is gone, but your teeth... aahh the pain. You have to ease the pain in your mouth by resting a mouthful of strong spirit at the painful part. It does help and you have some homemade herb liqueur you start to be quite fond of.
Nasty night pot
Doesn't matter what you eat or how it is spiced the night pot always smells awful, rotten and acidic. Most of the time is doesn't bother you and servants only tell tales about the horrible experience, but when on the road your companions will also notice and quite makes it easy to follow where you have camped.
Hangover doesn't seem to stop
After these many years even the smallest amount of alcoholic beverage gives you hangover. Everyone drinks watered down beer or wine as waters do give nasty illness... so please speak quietly and no sudden movements or your head will hurt.
Even in the summer high you feel cold. Maybe cheating death in a way brought you closer to the reaper than you have imagined. You ache the warmth of a fire and need good fur coat just to feel normal.
Food is no longer as it was in your childhood
It's all like ash, you lose hunger quickly and people have to remind you to eat because you don't really want to. You still feel if it's warm or cold food, slowly you become obsessed for foreign tastes. If you have never eaten it you can still feel it at least once.
Nothing is clean enough
Cheating death meant for you to start living cleaner. Getting the idea from Faeries or some Greek / Roman philosopher you are a bit obsessed: people are to wash their hands before touching you, always change to clean clothes couple of times a day.
Some of the above is not totally fit to historically accurate middle ages themed Sagas, but I think you get the grip.
Cheating the reaper is a way brings some minor part of old age closer, mental or physical.