"Transfer" applications may be filed at any point during the year, and do not need to go through the random lottery in March.
The only thing being "transferred" is the fact that the employee is exempt from the random lottery. Aside from checking a box on the application form to indicate this, transfer applications are identical to new applications.
USCIS considers every application to change employers to be a completely new petition. The company will have to prove that the particular job offered qualifies as a specialty occupation, and that the employee qualifies for this specific job. This is the case even if the job is almost identical to the one that the employee held with their prior H1B sponsor. For this reason, the level of detail required in the paperwork for a “transfer” application is exactly the same as that for a new one.