Hello I currently hold a non O single entry retirement visa with prior in country extensions . I would like to do some travelling outside thailand and wondering if I can change to a multy entry at my next extension but retain my access to do my 90 day reporting internally or do I have to leave the country every 90 days if I change?

You mean you have a 12 month extension of stay?  All you need is a re-entry permit, single or multiple.  You can buy that at the airport on departure or at immigration at any time, no need to wait for your next extension.

3 minutes ago, supermac said:

wondering if I can change to a multy entry at my next extension but retain my access to do my 90 day reporting internally or do I have to leave the country every 90 days if I change

You dont change extension to "multi" 

You obtain your 12 month extension as done previously.

You also buy a multi reentry permit.

That will allow you to exit and reenter Thailand as often as you wish.

The expiry of the reentry permit is the same as expiry of your permission of stay.

Multi reentry permit is 3800b.

You can obtain at your immigration office or at airport on day of departure.

Immigration at airport have optional service fee of 200b to fill in application form TM8 and photo.

 

Your 90-day reporting is cancelled when you leave and resets the day you re-enter with your permit. The re-entry permit is 1000 baht so it is not worth buying a multi-re-entry permit unless you are going to leave 4 times or more.

34 minutes ago, Jaggg88 said:

The re-entry permit is 1000 baht so it is not worth buying a multi-re-entry permit unless you are going to leave 4 times or more.

That's correct. Guess only thing to add is that a reentry permit takes half a page of pp so for me anything more than one exit I would obtain a multi..