@sezzo35 assuming that you have set up an entity as a partnership under US laws ( entities are usually formed under state laws -- so which State? Delaware ? ) To change your entity type you would need to contact that state to advise on how to achieve that.
(a) The EIN ( Employer Identification Number ) is attached to the entity and not to the type of entity-- it is a number used to identify the entity and among other things to credit the tax deposits of employees.
(b) To change the entity from partnership to sole proprietor, you need to talk to the State under whose laws it was formed ( see above )
(c) Your current entity type ( you need to correct this ASAP ) requires you to file two returns -- a business returns at entity level ( the date is as shown in your EIN application -- 03/15 /XXXX . Then there is individual return on form 1040-NR ( NOT supported by TurboTax -- See SprinTax or a tax professional ) covering your US sourced income and a flat 30% tax.
(d) Yes to file a tax return as a Non-Resident Alien, you may need ITIN.
VERY STRONGLY suggest that you see a US Tax professional , familiar with international filing -- at least for this first year you should not do it by yourself-- tax laws for NRA ( Non _resident Aliens ) are complicated and often impacted by Tax treaty between your country and USA.
