Yeah, you can swap any skills when you want, except when your in greater rifts. Dont forget also to turn Elective Mode ON in the option. Youll be able to use more than 1 skill per categories, and also turn advance tooltip ON too, give you more info about skills
Yep! Absolutely everything can be reversed. There's even an option to save your different builds as presets that you can switch between. There's a ton of equipment combinations that can drastically change how your character works and figuring out whether or not you want to keep legendary equipment long-term vs consuming it equip it's effects (using the Kanai Cube) as a passive is a major part of that. Save any legendaries you come across so you can figure that out.
Equipment prior to level 70 except for legendaries you want to extract the passives out are super unimportant, so really.. go wild and experiment!
Cool, thanks! Now I just need to choose a class I'll like, haha.