If there are existing leases on a property that you're looking at buying. You're required to honor them to the existing terms. It can be a lot of work transitioning new tenants and it can be pretty hard to determine if the current tenants are good or bad. One protection you can get is to require Tenant Estoppel Certificates. This essentially clarifies the tenant relationship in a legally binding way and can clear up any specially negotiated terms to existing tenants.
Once you purchase the property, it's important to reach out and provide clear instruction on where and how to pay next months rent. It would also be appropriate to have the previous landlord or management company notify the tenant of the change and give some instruction to them from you.
Tenants may need reassurance that leases aren't changing because tenant's can worry about the transition leading to them having to move, be kicked out, or have their rent raised.
It's important to get a copy of all lease and deposit documents so that if you make any changes it's not in violation of the existing lease. If the leases are currently on a month to month basis, then changes can be made with the required amount of notices.
If you want to make changes, we generally recommend making them slowly over the matter of a couple of months. Abrupt changes can lead to tenant turn around which can be costly.