No, you just have to go for it and reschedule them all. You need to set a whole morning aside, put on some coffee and get to work... The more you do this the quicker you will become. I assure you, rescheduling the routes for 20 aircraft is nothing when you are running a really big airline. I pretty much do that every morning before I go to work.
As you say, re-creating the new routes first might mean buying lots of extra slots, but are you aware that you can create new routes WITHOUT buying the slots? Just remember to check the little tick-box when creating each route. I suppose, in theory, you could create all the new routes without slots, then close the old ones, then assign the new routes to each aircraft, then just edit your new routes to pick up the slots. Personally this sounds messy to me and I don't bother. I could get 20 planes rescheduled in maybe 60 or 90 minutes - that means them only missing flying for 2 or 3 days in the game. You can at least plan it all out in Word or Excel or on paper first - know what times you're going to use and how long each route will take and then it shouldn't take too long to actually create them.
Also, as to your other question, NO your loads will no be affected by recreating the routes. The system recognises that you have already been flying the route and you should still get the same loads, even if you don't fly it for several weeks.