That's what I always do. If you have a +1 Lifedrinker, and you want to make a +3 one (or upgrade an existing to +3), just subtract the difference. A +1 weapon costs 2,000 gp., a +3 18,000 gp., so you have to pay the price in the DMG + 16,000 gp. for making one, or just the 16,000 gp. for upgrading one.
I don not allow 'reverse engineering' in downgrading, however. A frostbrand is always at least +3, and you cannot subtract x gp. to make a +2 frostbrand, because I think the market prices of specific weapons may not have been balanced with downgrading in mind.