Will my student visa still be valid if I do not travel when I originally intended to?
You must be in the US by the start date on your I-20 for your visa to be valid. If it is close enough to your start date you may show a letter of extension from your school at the Port of Entry or a new I-20 with a new start date. You will need a new visa if the new I-20 has a different SEVIS number, or if you have missed the original start date by more than 4 months.
If I want to go earlier than 30 days prior to the start date on my I-20, can I go in on my tourist visa, cross the border out, then come back in on my student visa?
Yes.
If I transfer to another school or from ESL to my university, how do I change the visa?
You usually do not need to change the visa. Make sure you have an updated and correct I-20 and that your designated school officials in both schools keep the SEVIS system updated. You will need a new visa if your new school has a different SEVIS number, or if you have been out of class for more than 4 months.