Yes, you don't HAVE to but certainly can ... and DO need to file 3115.
To know if you HAVE to change, add your gross receipts
for the most recent tax
year to the previous two years and divide by three.
If the average is over $5 million, you have to switch to accrual. Also, you have change if your business carries an inventory and your gross receipts are more than a million.
But yes, to change, complete IRS
Form 3115, (including the adjustment
from switching to accrual)
Attach your P & L and balance sheet for the previous year.
You can file at any time after the first day of the year for the year you want to change, including waiting to file it as an attachment to your tax return for that year.
Unless this is your last year of business, or your currently under an examination/audit
you can file under the "automatic change" process, which requires completing sections I, II and III of Form 3115. This gives you automatic approval if you meet the IRS requirements.
If your company IS an exception, again, your last year of business, or you're already under a tax examination -- you have to file for the "advance change," which requires parts II, III and IV.
Let me know if you have questions...