So far, it has not been as straightforward as I had hoped. The first problem that came up was that I had no copy of the Marlin configuration files from the original build. Ugh! I'd have to figure it out all over again. The next issue I had was that the original Arduino IDE installed on my computer was sorely out of date. When I updated to the new Arduino App for Windows 10, the libraries weren't compatible and every time I tried to compile Marlin in Arduino, it failed. After much research, it seems that the Arduino App on Windows 10 just won't reliably compile Marlin. The solution was to download the Visual Studio Code IDE with the PlatformIO and Auto Build Marlin add-ins.
Sadly, this didn't immediately resolve the problem, but it did provide very detailed logs that helped me figure out what was wrong. And I had a lot of things wrong. I entered decimal numbers for stepper motor counts when only whole numbers are allowed. I also selected the default motherboard "MOTHERBOARD BOARD_RAMPS_14_EFB" which was barfing on the second extruder settings. It turns out that EFB stands for Extruder Fan Bed, and a dual extruder machine is EEB (Extruder Extruder Bed). I fixed all that and finally got the Marlin to build and load onto the printer.
Since I was doing all this, I decided to also replace the original A4988 Pololu stepsticks with DRV8825 drivers. The DRV8825s are odd because the trimmer pot is on the opposite end of the board than the A4988, and the direction you turn the pot is also opposite. Another thing was that the pots were turned all the way up to the max, so the stepper motors got really hot as soon as I turned them on.
The next issue was calculating the step counts for the stepper motors. While the Prusa step count calculator was correct for the Z axis, it was pretty far off for the X and Y axis. This was compounded by a very high max feed rate and homing feed rates. It took me half a day to get the driver current, step counts, max feed rate, and acceleration settings.
After all of that, I decided to tackle another problem where the LCD was blank. I thought that I probably had the wrong display selected in the Marlin configuration file, but a little research lead me to the contrast setting on the display board. It only took a little tweak to fix that.
The controller fan wasn't working either after loading Marlin, which was also just a matter of a couple of lines in the configuration_adv.h file.
Things I have left to do: configure the extruder fans, wire up the extruders, build the cold ends for the Bowden extruders (waiting on the drive gears), mount the heat bed thermistor and run a few calibration prints.