I was sent a Coravin system some time ago and am also 100% converted. Great invention. Yes, I am not using this on a Fonseca 1948 and yes, I do not put my bottles back to the cellar, but drink them over time or give them away to be enjoyed by other persons.
I also think the Vintage needle is not great for old Ports as it is stuck sometimes, but the regular needle is great.
Nevertheless, apart from Coravin being a great system, with young wines and Ports I always fill them to half bottles or 0,1L bottles and put them aside, if I do not want to finish a bottle. Port was made to withstand unfavorable circumstance (like long sea travels with sun exposure) and is perfectly able to being re-bottled for the short term use (up to 6 weeks). I do not see any bad changes after rebottling. I am also doing this with Tawny Ports, especially older ones. Alex is correct that this Port-style has been exposed to oxigen for decades, but I still think that an open Tawny bottle looses a certain freshness and complexity.
Anything to do with Port.