I have been paid salery plus commission since being hired by company and without notice or warning I was informed that effective Aug 1st I would be paid commission only. Is there any law or regulation that prvents this? I asked employer for the changes in writing and was refused. All other personel are still being paid salery plus commission. Do I have any rights with regards to this issue?

On what date were you told this? Do you have an employment contract?

August 1st, No contract

Then yes, it is legal to change your pay structure going forward.

However, I do have a job offer letter. That spells out the compensation as follows.
Annual Salary plus % of gross profit + auto allowance.

"To be reviewed quarterly with the possibility of going to commission only plan that provides additional reward for higher profit sales at a mutualy benificial time".

Which only weakens your case as you were told up front that this might happen.

An offer letter does not normally rise to the level of a binding contract. You can have it reviewed by an attorney if you wish, but I would not be optimistic.