It's impossible to put down a Mach 3 and pick up a straight razor and make the transition. It's not an either/or proposition. Get some practice under your belt and then start using the Mach 3 less.

Don't put any pressure on your self. You've got a few years before Gillette discontinues the Mach 3 and a few decades before the world's disposable steel reserves disappear. THEN you'll be forced to transition completely.