You can switch to a different paradigm at any time, but you would be better off to at least wait until the current turn is over. If you still have commands being executed, they will be canceled if you change to a different paradigm. When you switch to a new paradigm, the ATB bar is either reset or completely full.
If you have been in the current paradigm for at least two turns and switch to a different one, then the ATB bar will be completely full for your entire party. For this reason, I tend to spend at least two turns in each paradigm. Sometimes I switch to a new paradigm not because the current one isn't working, but rather I want to get a full bar and so I switch to a different paradigm that would also work for the current fight. Actually, if you switch to a different paradigm on every turn, you will get a full ATB bar on every other paradigm.
If you don't like to use that many different paradigms, you might consider creating two copies of your fighting paradigm (I tend to use Relentless Assault) next to each other. You can then spend two times on the first paradigm, switch to the second one for two turns, and then back to the first. Why would you do that? Because each time you switch you will have a full ATB bar, which means no wait for that attack. Not as good as haste, but a help.