I have finally managed to reprogram the original Moteino in Moteino 8MHz.
It wasn't an easy path, so I would to share how I did it.
First, I didn't use the Arduino IDE, as it's mostly a black box.
One issue is the older programmer "AVRISP mkII" from Atmel is discontinued, but the good news is there is a perfect copy of this older programmer made by Waveshare (model "USB AVRISP XPII"):
https://www.waveshare.com/usb-avrisp-xpii.htmI bought it from Amazon and it's working perfectly.
Once you have the programmer, you need (on Windows) to use the "Zadig" software to force the device in "libusb-win32" mode.
You can download a portable avrduded version for Windows from here:
http://download.savannah.gnu.org/releases/avrdude/I have used the "avrdude-6.3-mingw32.zip" file, that is containing the executable plus the configuration file for using the different programmers.
To erase then upload the correct bootloader for the Moteino 8MHZ (with the correct fuse options), this command line is working:
avrdude.exe -C avrdude.conf -p m328p -c avrispmkII -v -e -U flash:w:Dualoptiboot_v5.0_atmega328_8mhz_57600baud_BlinkD
9_SPIdisable.hex -U lfuse:w:0xD2:m -U hfuse:w:0xDC:m -U efuse:w:0xFE:m -U lock:w:0xCF:m
I made a breakout board in order to add an ICSP connector (HE10 with 6 pins) and also to provide a pull-up option (10K resistors) for the Chip Select (CS) of the SPI flash and RFM69 module that are installed. Check the attachments if interessed.