It's going to take some time to get back into your old groove,if anything you'll have a new one. Embrace it and have fun man,at least you're trying.
Agreed, the way you skated 15 years ago versus today is night and day, probably more so if you took a few years off the board. It's probably really hard to completely relearn years of muscle memory and wouldn't be worth the effort given how much less time we have on the board as adults. Making small changes to your body (stance, foot placement, shoulders, head position) every time you try a line or trick is probably the more viable way to go.
Board size preferences have also changed a lot in the past decade. I find trying to mix a modern board (8 inches and up) with older truck designs (lower, smaller, heavier), doesn't work as well. If you can afford it try and toy around with your truck setup; adjusting your wheelbase can drastically alter how it turns, pops and feels weight wise.