I won't claim expert status, but I do have a little experience skiing both old and new types of skis, so perhaps the following observations are pertinent.
If your old boots aren't packed out and fit snugly, then give them a good whack with a hammer. If the hammer doesn't shatter them, put them on and flex them, one at a time as hard as you can. If they don't fall apart, you should be good to go.
If you could ski your old skis when you put them up, you can ski them the same way now, or in about 20 minutes on the slope, after getting used to them again.
Changes required in technique to adapt to new skis very much depends on how you skied the old skis, and which new skis you get. If you were arcing SG and ripping GS turns on the old skis by tipping them onto their edges, then you can carve turns the same way, but the turns will be tighter, a lot tighter if you get a SL sidecut. One key difference is if you want to carve a fairly tight turn you used to have to decamber the front of your ski before tipping it and making it dig in; with the more shapely modern skis you just have to tip 'em and even if you keep your weight centered on the ski it will decamber automatically into a turn. Nothing could be easier; tip skis right to go right; tip skis left to go left.
Some skis will allow you to use all the old moves without complaint, some would prefer you keep the edges locked in. Most of the "race" models prefer not to let the edges skid, though you can still force them to do so. Most of the general purpose mid-level all mountain skis don't care, but don't give back much in the way of performance.
What to get/demo? Depends on your skiing: If you like to ski fast, get a gs ski or ski with a gs-like radius (21+ m), make a lot of SL turns at speeds high enough to pull a couple of G forces, get a sl ski. Ski slowly get a recreational ski with a SL (about 13 m) radius. Ski powder, get a powder-specific ski. Best bet is to rent/demo and get the best suited ski for everything you do without having to buy a quiver.
Definitely have new skiers start out on a good SL radius ski, but not a racing SL ski (requires too much speed to bend nicely, and things happen too fast for newbies to handle and learn from at that speed).
Hope that helps.