Yes, you may freely change and wash your احرام garments — the state of احرام is maintained by your intention, not the specific pieces of cloth.
Yes, changing احرام garments is fully permitted and does not affect your state of احرام in any way. The state of احرام is a spiritual commitment entered through intention (niyyah), not a property of the physical garments. You may change to a fresh set of احرام cloths whenever you wish — whether because the current ones are dirty, wet from sweat, or simply for comfort. Many حجاج bring 2-3 sets of احرام garments for this purpose.
You may also wash your احرام garments during the حج. Use unscented soap or detergent to avoid violating the perfume prohibition. Washing and drying احرام cloths is perfectly permissible. Some حجاج wash their garments in hotel rooms and hang them to dry overnight.
The only requirements when changing are that the new garments must meet the same احرام specifications: for men, they must be two unsewn, unstitched white cloths without any perfume or fragrance. When changing, be mindful of the head-covering prohibition for men — do not accidentally pull cloth over your head while removing or putting on the rida (upper garment). The process of changing should be done modestly, preferably in a private area. Carrying a spare set of احرام garments at all times is مستحب, especially during the days at منیٰ and عرفات.
Source: Ibn Uthaymeen, Majmu Fatawa, section on احرام