Peanutz, If I was you I won't send the B.E. a letter about switching your fiance to spouse, you're making them confused they will also think that you and your partner didn't plan everything properly before submiting the application to them. This is a big mistake indeed!
They might just going to send your fiance application back and let you submit the application you wanted which is spouse but I'm sure you'll have to pay another fee again for the spouse visa.
Hope you can solve your problem soon. Goodluck!
