Inmmigration's website for BKK says how to change a Tourist visa to a type O, but the only one likely to be a Volunteer Visa is a visa called a Type O Foundation Visa (number ten on the conversion list). Here is the link.

You may get more helpful suggestions if you reveal what you're planning to volunteer for, and whether it's an approved charity, etc.

 

1 hour ago, FangFerang said:

Inmmigration's website for BKK says how to change a Tourist visa to a type O, but the only one likely to be a Volunteer Visa is a visa called a Type O Foundation Visa (number ten on the conversion list).   Here is the link.

 

Any help would be appreciated.

Do you have some problems with your eyes or forgot your glasses requiring you to type in massive font size rather than normal font size?

@FangFerang I have changed your font size.

Also you mention link but was not posted.

You need to provide more information and asking for specific advice  as per @impulse post