Not sure why you want to switch. You've already put new tires and tubes on it so you should be good for a while. 27" tires are still available - the selection is meager but you can still get them.

The lowest of the low end late 80's Japanese road bikes still used 27". I have a 1988 Bridgestone RB-4 which has 27". It was spec'd at 23 pounds, and yours is 26 pounds - it's a department-store quality bike.