You possibly have stem shoplifters so the only thing you will need other than the new straight handle bar will be brake levers to match your brakes, almost certainly need to shorten the brake cables as well. & of course new handlebar grips.

If you have not looked into it there are a considerable amount of slight variations of a few degrees in the various types of straight handlebars, so it will be worth having a good look around at the variations in design, these type of handlebars also come in a selection of different widths, so you need to think about that, to which will be more comfortable, yes some can be cut down but not too much as handlebars maybe tapered towards the clamp area, & you then cannot slide the brake levers on to where you require them.

Downhill bike handle bars tend to be of a heavier gauge & much wider which you may not want.

There are also two main diameters of handle bars so you need to measure the clamp area on your to make sure you buy the same.