@Angelina ****
I think you're confused a bit
A single entry tourist visa is VALID for 90 days from the date they stick it in your passport <- that means you have that long to ENTER thailand when you'll be stamped in for 60 days like everyone who enters on a 'regular' tourist visa.
And
Yes, many people have entered on a tourist visa and converted it. There are no problems getting it converted to either a Non-ED visa (for study) or a Non-B visa (for employment) IF the place you're studying at OR working at can meet the requirements at the immigration office to get that 90 day Non-Immigrant visa issued