Nothing happens to the period value. Thats always in seconds.
If you change from ramping up from 0 - 20/sec to ramping up from 0 - 20/min than that's what will happen over the prescribed period. So I think if you did something like 0-20/min over 30 seconds then after 30 secs you will be at a rate of 20/min. Don't forget that the test can run for longer than the 'period'. Thats just the ramp up period.
Are you talking about changing to mins or hours part way through the test?
In terms of simulating users, it really depends on what you expect them to do. They may not do it in a uniform fashion as your generator will produce but so what. If you load your system to find it's limits then you know what rate of requests it can handle.