Yes, you can change to local Bhutanese currency at the airport upon arrival in Paro, or at your hotel, Bank etc. However Nepalese currency is not accepted in Bhutan and so you should not bring Nepalese currency. Indian currency (with exceptions of large bills such as Rs. 500 ) can also be used. At the end of the trip, you can change any left-over Bhutanese currencies into Indian rupees locally from local shops etc. It is generally not very difficult to change back. If you have saved the receipt for monies changed, you can also change back to US$ or currencies used in the beginning, if you go to the bank. As to how much you need to change, it is very difficult for us to guess but I would approximate to around $20-40 per day per person for out of pocket expenses if your trip includes all meals. You will need money for drinks, beverages, tips etc. However you can always change some money during your travels if you need one.