So it is your 1st time applying a WP under this BOI company? For renewal, I don't think you need a letter anymore. It is always easy process to renew, unless the company has some report delayed to the other government department. I can be wrong though, I do not have direct experience with BOI, only observed info from others.
If you are here in Thailand and do not want to leave the country, yes you need a NonB first which can be converted from other visa type. The info I mentioned, is for applying a Work Permit from overseas.
In general and if you have all document ready. You can have a staff apply for your work permit in Thailand for you. Within 2 weeks you should receive an approval of WP3 form. Your staff must courier this approval to you. Then you can take it for a NonB visa application in your country. Once you are back into Thailand, you can complete your work permit process (another 2 weeks).
Note, the approval of WP3 form is only valid for 1 month.