Hi Jay,
I believe it is fine to change your NOC after receiving an ITA, so a LOE is not mandatory. However, if you want to include one just in case, you could write something along the lines of:
Dear Officer,
I am writing you this letter to explain my selection of NOC for my work experience as [job title] at [company name]. Between the time of receiving my invitation to apply and submitting my application, I decided that NOC [####] is a better match than NOC [####] for this work experience. [You could also add an explanation why you feel the new NOC is a better match than the previous NOC but, again, this is optional].
I am of course at your disposal to provide you with any additional documents that you deem useful for the processing of my electronic Application for Permanent Residence (e-APR).
Thank you in advance for the time devoted to the treatment of my e-APR.
Sincerely,
[Your name]
I hope that helps!
