I just updated my Maker Select Plus from the stock (I believe RepRap-based) firmware to Advi3pp, which is Marlin based. The printer starts up and everything seems okay, but I haven't actually tried a print yet and there was a message during the upgrade about deleting incompatible settings.
What do I need to do to recalibrate the printer following the firmware upgrade?
If it is Marlin based or RepRap based, many parameters are stored in EEPROM memory. A G-code command M502: Read parameters from "configuration.h" would reset all parameters that can be changed to their default value as defined in your configuration file. Don't forget to follow the M502 command with a M500 command to store the loaded parameters to EEPROM. This would overwrite all previous settings.
From the linked source, M502:
This command resets all tunable parameters to their default values, as set in the firmware. This doesn't reset any parameters stored in the EEPROM, so it must be followed with M500 if you want to do that.
You can send these commands over a terminal interface to the printer using applications such as Pronterface, OctoPrint, Repetier-Host, and probably many more, or store the commands in a G-code file (e.g. a text file with a .g extension) and print the file using an SD card.
Looks like I don't need to do anything. I printed a 20 mm calibration cube, and aside from some elephant footing it came out as clean and as close to the model dimensions as anything else I've ever put through the machine, with no changes.
So I'll recommend this as a first step to anyone else: start a 20 mm cube going, watch it closely early on to be sure you're getting adequate extrusion and bed adhesion. If it fails here you may need to adjust settings. When it's done, measure it and see where you are. You might not need to do anything else.