Hmmm...ok, I am wondering if I am not better of just scraping them or relegating them to defense of earth; kind of a mobile pdc or something...refit looks long, painfull and expensive.
You'll have to judge for yourself. Once you're retooled for the new class, you can compare the cost of building a new ship to the cost of refitting from an old one. For me, anything over 50% makes me think about whether I'd be better off building new and keeping the old ship as a 2nd-line unit or scrapping it. At present, I tend to leave ships that are one generation out of date as 2nd-line units, then scrap them or upgrade them when they're two generations out of date.
John