I am the recent victim of H1 to F1 and got denied. There is a valid reason for denial. However, I have learned a lot especially what all are required and processing methods and all. THought I would share my story who are in same situation of H1 to F1 and right way of doing it.

Usually H1 to F1 is more likely or easy task if applied on time with complete documents.

Following documents are needed
1) Fill the form I-539 find online on USCIS website
2) Payment to USCIS the fee is $300 ( last time I applied)
3) Original I -20 signed by International student advisor and clearly indication of SEVIS bar code
4) Original I -94
5) Copy of passport Pix page and info page
6) Copy of passport visa stamping if any ( for H1B)
7) Recent paystub (last 3 - 4 ) transactions if not working in between need Leave of Absence or some kind of letter from Employer.
8) Most importantly bank statement showing enough money to cover at least whats mentioned on I-20
Usually the best is to show more than what is shown on I-20. For example , If I -20 says $16000 / year  I would want to have at least $20,000. If I showed just enough ( eg. $17000) then they would ask me to show last few transactions just to show there is consistency over those balance.
9) Need to pay SEVIS I-901 fee at online https://www.fmjfee.com/i901fee/. and provide the copy of payment.

Once u get the I-20 you should immediately apply for change of status. I would say about 3 months before the classes start. IT's good to apply early even though you can continue or start classes while the case is pending. But who knows if case is denied or so. USCIS normal or maximum processing time for this is 2.5 months if documents are completed. Otherwise there might be RFE or so and it will lengthen the processing times.

Hope this helps.

Thanks
Y