Can I convert from Rithmic to Tradovate or vice versa?

No, the account setup is different and each has their own fees, you must sign up for a new plan.

How important is it that I sign up for the right plan?

It is PARAMOUNT that you choose the right plan on the web page or in the dashboard if you choose the wrong plan or the wrong platform there will be NO REFUNDS, no exceptions. So be careful; we incur charges for every new account setup and can not issue refunds for user mistakes. Read before you click!