Hey, welcome back.
To put a finer point on what [MENTION=97077]iserith[/MENTION] said: the fact that 5e looks, at first glance, like AD&D is a snare and a delusion. I mean, it's nice that it feels 'comfy', but it definitely, definitely, definitely does not play the same way.* My history is very similar to yours, and I immediately instituted some house rules based on my memory of how 1e played. While I wouldn't call the result a disaster, it soon became evident that what I had done was, at best, silly.
As one example of something that deserves some contemplation - look at the resting / resource recovery rules, think about how different they are from 1e, and think about how that will affect the game.
Another thing I'll mention: it's sort of a meta consideration, but unlike 1e, 5e is a game that was actually designed based on several decades of experience with RPGs, as opposed to something that emerged organically from the primordial soup of wargaming and fantasy literature. So while it would probably be an overstatement to say that 5e has design principles (as in design rules that were strictly adhered to), it does have systems that are intended to hang together in fairly specific ways. Unfortunately, any explication of these considerations/guidelines is pretty much entirely absent from the books, even the DMG. The upshot is that mods can have ripple effects that actually do degrade things that were working well, as opposed to 1e where the interaction of different systems was so idiosyncratic that all you could say about most changes was, "Well, that's different."
Anyway, even though you will see it said that 5e is very modifiable, my advice is to spend significant time playing RAW before you start tweaking. And don't worry, there are still plenty of holes and edge cases that you will have to fill in with rulings, so your creativity will not be stifled.
Finally, [MENTION=56051]Raith5[/MENTION] made a humorous reference to studying the intervening D&D editions. While it is by no means critical or a prerequisite, I have found learning a bit about 3e and 4e instructive. Elements of those games have influenced aspects of 5e.
* Though I did not play 3e or 4e, from what I have gathered, 5e does seem to be more reminiscent of 1e than 3e or 4e were, but that also seems to be a low bar.