uPD4711 to 4724 (RS-232 Driver/Receiver)

What is the role of the external capacitor of the uPD4711?

This is a step-up capacitor to generate the required 12 V for a circuit from a 5 V power supply voltage.
A higher voltage than the power supply voltage is achieved by charging the capacitor with electricity and then using the voltage stored in that capacitor.

Can the uPD4711AC be replaced with the uPD4711BC (DIP type) just by replacing the IC?

The difference between the uPD4711 A and B is explained in p.35 of User's Manual (S13354*): How to Use IC for RS-232 Line Driver/Receivers.

The PDF file of the User's Manual (S13354*) can be downloaded from the following URL.
How to Use IC for RS-232 Line Driver/Receivers User's Manual

What is the guaranteed data rate of the uPD4721?

The data transfer rate of the uPD4721 is 20 kbps maximum.
It mentions in the User's Manual.

The PDF file of the User's Manual (S13354*) can be downloaded from the following URL.
How to Use IC for RS-232 Line Driver/Receivers User's Manual

Currently, a 3.3 uF aluminum electrolytic capacitor is used for step-up purposes, but I want to replace it with a ceramic capacitor with the same capacitance. Would that be a problem?

A ceramic capacitor can be used without problem. Refer to the following documents.

Excerpt from p. 25:
Selection of external capacitors

Tantalum, aluminum electrolytic, and ceramic capacitors can be used for external capacitors for on-chip DC/DC converters.
Since they are repeatedly charged and discharged by internal switching, use capacitors with better frequency characteristics.

It is recommended to apply capacitors with a capacitance range of 4.7 to 47 uF (for the uPD471X Series, excepting the uPD4711B with capacitance range of 1 to 47 uF) and 0.33 to 4.7 uF (for the uPD472X Series, excepting uPD4722 with capacitance range of 0.47 to 4.7 uF).

Determine appropriate capacitance within these ranges after evaluating with the actual product circuit.
Note that the capacitance of electrolytic capacitors is lowered in low temperatures. Therefore, determine the capacitance of such capacitors with some margin taking into account the operating temperature.

If only 5 V Mode (VCHA ="L" and VCC = 5 V) is used, it is not necessary to connect the C5 capacitor. In this case, the C5 pin should be left open (for uPD472X Series).

Reference document:
How to Use IC for RS-232 Line Driver/Receivers User's Manual

The ability values of the uPD4722 are listed in "Chapter 12 Q&A, 12.4 Transfer Rate" in How to Use IC for RS-232 Line Driver/Receivers User's Manual, but how about the uPD4724?

There is no measurement data for the uPD4724, but the transfer rate performance is equivalent.
Therefore, refer to and use the data for the uPD4722 listed in the User's Manual.