For backwards compatibility, G26 now calls M999

Marlin/Conditionals_LulzBot.h

@@ -115,6 +115,18 @@
 #define LULZBOT_EEPROM_SETTINGS
 #define LULZBOT_EMERGENCY_PARSER
 
+// Prior branches of the LulzBot firmware used G26
+// to reset a probe failed condition. Marlin upstrem
+// now redefines that for unified bed leveling. The
+// following maps G26 to M999 for equivalent behavior,
+// so long as UBL is disabled.
+#define LULZBOT_G26_BACKWARDS_COMPATIBILITY
+
+// The following should be kept more or less:
+#define LULZBOT_G26_RESET_ACTION \
+  Running = true; \
+  lcd_reset_alert_level();
+
 // Temperature settings
 
 #define LULZBOT_TEMP_SENSOR_0                 5

Marlin/Marlin_main.cpp

@@ -10521,6 +10521,10 @@ void process_next_command() {
         case 26: // G26: Mesh Validation Pattern generation
           gcode_G26();
           break;
+      #elif defined(LULZBOT_G26_BACKWARDS_COMPATIBILITY)
+        case 26: // G26: LulzBot clear probe fail
+          LULZBOT_G26_RESET_ACTION;
+          break;
       #endif // AUTO_BED_LEVELING_UBL
 
       #if ENABLED(NOZZLE_PARK_FEATURE)