The most important thing (imho) is to not change your bedtime routine at all. Only change the actual bed, lol. Do everything else exactly the same. If you want to make a "big deal" out of him getting a big boy bed, be careful that you don't make such a big deal that it creates stress for him.
We also put up a guard rail on the bed... didn't ask my son's opinion, just put it there like it was part of the bed. Push one side of the bed right up against the wall and use a guard rail on the outside side. That seems to help them still feel "cozy" and keeps them from accidentally rolling off the bed, AND helps deter them getting up out of the bed after you tell them goodnight. :)
You can let him help you pick out a new quilt or sheets, too.
Do you plan to let him keep his room? Or are you moving him into a new room (along with a new bed)? If possible, I would put the new bed in the same room he is currently in.
Ooohhh... I just remembered. We, too, kept similar type bedding. He had Winnie the Pooh stuff and so I bought pale green window pane sheets and a pale blue/green bed spread that practically matched his Winnie the Pooh stuff. Then continued putting the WtP crib comforter on top of the bed. He also had some blankets that were crocheted by my mom that he loved, and we kept using those also.
We didn't have any transition issues at all. All the problems with him getting up out of bed came long before the big boy bed, lol. (He was crib climbing by one year, and so his crib mattress was on the floor fora good 6 months before putting in the twin bed).