I would write them a letter and meet them in person. If you can't meet them in person soon, write and send the letter as soon as you can; if you can meet them in person (say) this week, write the letter and then give it to them in person.
The letter doesn't have to say a lot, other than the new way they should pay rent, and that they should get in touch with you if there is a problem. Maybe something like this:
"I am taking over the management of the property at 123 Main Street. The method of making your rent payments will change, starting with the payment due on January 1, 2020. You should now make checks payable to Morgan Gruelle, and mail them to PO Box 567, St. Louis. Alternatively, you can pay electronically, through the website Cozy.co (or Zelle or whatever). I will contact you in the next few days to discuss the electronic payment option.
The rent amount and rent due date have not changed. The lease terms have not changed; your current lease is still valid, and expires on (April 30th, 2020 or whatever date).
Starting January 1, 2020, if you have a maintenance request or another concern, please call or text Morgan at (phone number), or email (email address). Before January 1, you should contact XYZ Property Management, as you do now."
When you talk to them in person, they may ask if you're selling the building, or getting ready to kick them out, and things like that. Have an answer ready if they ask.
If you've got copies of the leases they have now, see if there is any language in there about changes to the payment methods, etc. Sometimes there is a requirement for an advance notice of changes. If that notice period is more than about 20 days (as of today), you technically can't yet change something that will happen on the 1st.