Pre-T (well, a month on T technically) I have a pretty male shaped body other than the fact that I am a proverbial twig (and my feminine waist is totally hidden by any shirt making it look invisible because I am thin), so this may not be super helpful, but I am also a personal stylist for a living, and I have worked before with cis men with "unusual" body types (not actually unusual but not the body type strived for) who are everything from short to curvy. Truth is, even most cis men cannot wear "whatever they want" and actually look good. Without a detailed description of your body type it is hard for me to list off denim cuts that are best for you, but you can absolutely wear jeans, though I would suggest investing in only a couple of new pairs and going ahead and saving the others as long as you don't need the space.
The key to finding clothing that highlights your best features is to be willing to try on DOZENS of styles and cuts without starting to feel depressed and like nothing will ever fit. (This is easier if you have a stylist because we are trained to look at body types and know what will work, but stylists are pricey). A lot of FTMs have the mistaken idea that there is a natural male shape. Actually, there is an IDEAL male shape. Real men are all over the board, and you CAN find denimstyles and cuts that look good pre-T--especially if you are willing to get stuff tailored. But I wouldn't hold off on buying all clothes until changes set in, simply because flattering clothes increase confidence. Similarly, I wouldn't get rid of jeans you love until your body shape settles in--but don't count on them working for you again. The fact is, every body shape has its imperfections that need to be hidden, and it is highly unlikely those will be the same for you after t that they were before. I personally prefer styles called "slim tapered." These are not tight like skinny jeans but they are not so relaxed that they look wide on my small frame. They are slim on the waist and hips and thighs (which are wider than my calves) then taper on the legs, continuing to hug the lines of my body. Outlet malls are an awesome place to go for jeans. There are SO many cuts of denim out there, I am 100% sure you can find styles you like now, and definitely save those old jeans just in case!
Edit: and btw, the tailoring thing? Know that ALL movie stars and models wear tailored clothing. Also most fashionista types get at least minimal tailoring. Particularly male fashionistas. NO ONE actually looks as good off the rack as magazine pictures show. So keep that in mind when you try on clothes. Tailoring is the secret of stylist--we hook clients up with awesome tailors.