A year ago I ported my project from Uno with esp8266 (Uno WiFi Dev Ed) to Wemos D1 R2.
Wemos D1 R2 board has esp8266 as the only MCU. The AT firmware is an application running in the esp8266. An Arduino sketch with esp8266 boards package is an application too, so it replaces the AT firmware.
Instead of AT commands you will use for networking the WiFi library of the esp8266 arduino package. It is much simpler and there are many examples.
The esp8266 and the D1 R2 board have 3.3 V logic level on pins. So be careful with inputs and check if your devices will read the 3.3 V as HIGH.
In code use Dx constants matching the labels on the board. D1 R2 has less pins then Uno. I recommend to use I2C devices if possible. I had a Grove Base shield on D1 R2, so I have a mapping of D1 R2 pins to 'Uno' pins.
Some of the esp8266 pins are read at boot as boot configuration. Pins D3 (io 0)
and D4 (io 2) have pull-up on board and pin D8 (io 15) has pull-down on board. Not all devices will work on this pins. Here is a pin overview of the NodeMcu and Wemos boards.
The esp8266 has only one ADC pin. The A0 of the esp8266 can read 0 to 1 V. Wemos boards have a voltage divider on A0 so the board's pin can read 0 to 3.3 V. Don't read A0 in every loop. WiFi part of the esp8266 uses the ADC hardware too and continuous reading of A0 interferes with WiFi operations.
The Wemos D1 R2 has a strong power regulator 7 to 20 V for Vin and power jack. You can power devices from 5V and 3.3V pin. I use Grove relay modules powered from 5V pin and controlled by logic signal at 3.3 V.