As I understand one does not need to leave Thailand to change a tourist visa into a non-immigrant visa. The granting of change of type of visa and extension of stay is at the discretion of the immigration officer.

However, I have heard that you must have 21 days left on your visa for this to be even considered.

Does anyone have any concrete information on this?

Thanks.

Only if you meet the requirements - basically that you are qualified to extend your stay on a yearly basis after the change is made.

21 days should remain but down to 15 I believe is still possible.