Small random-orbit or pad sanders make this job go faster. (Wallis first covers these boundaries with Synkoloid patching compound so no edge is visible after sanding.) As shown, you want to make sure that there is a feathered, smooth transition from exposed wood to old paint. For areas that might get close scrutiny, you can follow up with a 100- or 120-grit rubdown to erase any scratches.
Interesting information! We just got scammed in White Stone, Virginia... we chose the same color, but went from a flat to an eggshell finish. I wasn't available to stand over the painter while he painted. For such a detailed job, I marveled at how quickly he finished the project! After he was paid for the job, we discovered all he did was roll paint across the walls and close to the crown, baseboards and detailed trim around windows and doors leaving about an inch or so of the flat finish. By using the same color, he didn't even do the job he was paid to do, which explained why he could finish the job so quickly. By using the same color... he got lucky and passed it off as a completed job not bothering to paint to and cut in around the trim. Upon further evaluation of our walls we could see exactly where he stopped because we could see the difference between the flat and eggshell finishes. There are walls he didn't even bother to paint. Then where he did paint near the crown when we were in the room watching ... he hit the crown moulding and tried to tell us it was already there... and tried to sell us on painting the crown moulding. I am so disgusted and upset!! He'll be hearing from us to rectify the situation.
to be the devils advocate i have been a building contractor 20 of the last 30 yrs. i do know that if you go to a higher sheen of paint and or darker colors then any imperfections in the walls will show up much more dramatically…therefore the painter or a good drywall finisher is needed to prepare the walls extensively. this could cause more expenses…for it takes a lot of time to prep walls (smooth walls..not textured walls) and this cost has to be absorbed.