Well... I do the basement first...playfield last. I have gotten in the habit of putting Molex Plugs on the major components (Sling Shots, pop bumpers, kickers, etc...basically everything with a coil) so they can be removed. I replace all wire braid on the bottom of the playfield with 1/8" flat braid...similar to the heavier 1/4" ground braid that runs through the cabinet. For High Speed, it's a good time to add some simple modifications. The roll overs were never illuminated, however, the GI lighting braid are conveniently right there...so add sockets and connect to the GI circuit. Also a good time to add a light under the Escape Hole. While the playfield is pre-drilled for the most part, I generally re-drill with a slightly larger bit that fits the screws much better...avoids mushrooming, especially on the top of the playfield.
Check fit everything before you trust the "dimpling" on the playfield. You will often find they can be off a little.
Now here's a big one... The roll overs undoubtedly got some clear coat in them and on the CPR website are recommendations for clearing them....HOWEVER.. just clearing the vanes is not enough. You should have received three new stars with your game. Once the stars are inserted into the roll over they cannot be removed without doing damage to the insert. So, on your old playfield, on the underside of the roll over, take a pair of cutters and cut off the nub on the end of the star. This will allow you to remove it out the top of the playfield. You can now use this star to check the fit into the new roll over. Getting the vanes clear is a no brainer as the star must freely move up and down. There is a serious issue with whether or not the star, when depressed, goes down far enough not to interfere with ball travel. There could also be clear down in the insert on the shoulder that the star will come to rest on when depressed. On my game I did clean that out a little, but it was still insufficient to get the star to go down far enough. So, what I had to do was to modify the star itself. Where the star comes into contact with the shoulder down in the insert, I removed about 1/16" of material on the underside of the star around the shaft. You can experiment with the test star until you find out how much to remove to insure the star will go down even with the playfield. If you don't get the stars right, especially the right one, your orbit shots simply will not work as the ball will hit the star and fly upward... Whew...hate to rant on about this one but, it is a huge issue.
Beyond that, take lots of photos. If you run into anything you are unsure of I am glad to help. I did my HS swap just a few weeks ago.