ESP32 on 3 x D alkaline batteries?

[botbuilder]

I am working on updating a client's product design. It currently uses a 3 x D battery pack to power a 8 bit PIC and a micro servo.

I would like to update it to a ESP32 for its BLE capabilities as well as other capabilities.

What specific options (converters or regulators) do I have to efficiently power the ESP32 on the current battery design?

Thanks!

[ESP_Sprite]

It depends. I'd personally go for a buck converter; the hazard there, however, is that when the batteries get empty, their voltage may sag below the voltage where your buck converter can still work with it. If any, I'd put a big capacitor over the D-cells, as their response to current spikes generated when e.g. the ESP32 does its WiFi calibration may be to drop the voltage so low that the ESP32 resets. A large capacitor can help with that.