How do I begin the process to change salaried, exempt store managers to hourly, non-exempt store managers?

Since you say "managers" as in more than one, you hire a labor attorney to first see if it is possible, since their job classification has been salary and they have been classed. Also since you are not changing their job duties,

You are free of course unless there is a contract, to charge their pay rates, change their benefits and more.

Also there is a set amount, they have to be paid to be considered real salary, by federal law. So if you were not already paying that amount, you could even owe them overtime for many previous years.

You are allowed to change anyone to hourly. The FLSA has specific rules to classify employees as exempt salaried. You do not have to classify them as such if you do not wish to take the exemption from the FLSA. This is simply and exemption from overtime.

If you wish to pay them hourly and pay them overtime for all hours worked over 40 than anyone can be paid on an hourly basis there is no restriction.

Shirley