@Missy ******
Go to the 2nd floor of promenada immigration open from 10am-4. And ask them for papers information and schedule appt to hand in your change of application ( this will usually be scheduled 3-4 weeks later as they are backed up. Gives u time to get all.ur papers together ext. ... *** YOU MAY need t get an extension on your current visa before doing this is you dont have min 30 days on ur visa... u will probably end up needing one anyways as usually will need about 50 days all said and done.
so be prepared for them to send you downstairs to first get an extension.**
.go back on said appt date and hand in all documents. The immigration will go over each paper and certify and u will sign each paper with them when u are there. Pay 2000 baht. And u will get another appt slip 21 days later to go back in to get visa in passport ( u will go at 10 am to hand in passport that day and collect again 3 pm with visa in passport .