diff --git a/src/fios_gui.cpp b/src/fios_gui.cpp
--- a/src/fios_gui.cpp
+++ b/src/fios_gui.cpp
@@ -516,7 +516,7 @@
 				break;
 
 			case SLWW_LOAD_BUTTON:
-				if (this->selected != NULL && !_load_check_data.HasErrors()) {
+				if (this->selected != NULL && !_load_check_data.HasErrors() && (_load_check_data.grf_compatibility == 0 || _settings_client.gui.scenario_developer)) {
 					_switch_mode = (_game_mode == GM_EDITOR) ? SM_LOAD_SCENARIO : SM_LOAD;
 
 					const char *name = FiosBrowseTo(this->selected);
@@ -665,7 +665,7 @@
 				/* Selection changes */
 				if (_saveload_mode == SLD_LOAD_GAME || _saveload_mode == SLD_LOAD_SCENARIO) {
 					this->SetWidgetDisabledState(SLWW_LOAD_BUTTON,
-							this->selected == NULL || _load_check_data.HasErrors());
+							this->selected == NULL || _load_check_data.HasErrors() || !(_load_check_data.grf_compatibility == 0 || _settings_client.gui.scenario_developer));
 					this->SetWidgetDisabledState(SLWW_NEWGRF_INFO,
 							!_load_check_data.HasNewGrfs());
 				}
diff --git a/src/settings_type.h b/src/settings_type.h
--- a/src/settings_type.h
+++ b/src/settings_type.h
@@ -111,6 +111,7 @@
 	bool   show_date_in_logs;                ///< whether to show dates in console logs
 	bool   newgrf_developer_tools;           ///< activate NewGRF developer tools
 	bool   ai_developer_tools;               ///< activate AI developer tools
+	bool   scenario_developer;               ///< activate scenario developer: allow modifying NewGRFs on an existing game
 	bool   newgrf_show_old_versions;         ///< whether to show old versions in the NewGRF list
 };
 
diff --git a/src/table/settings.h b/src/table/settings.h
--- a/src/table/settings.h
+++ b/src/table/settings.h
@@ -617,6 +617,7 @@
 	  SDTC_VAR(gui.developer,                 SLE_UINT8, S,  0,     1,        0,        2, 0, STR_NULL,                                       NULL),
 	 SDTC_BOOL(gui.newgrf_developer_tools,               S,  0, false,                        STR_NULL,                                       ReinitWindows),
 	 SDTC_BOOL(gui.ai_developer_tools,                   S,  0, false,                        STR_NULL,                                       NULL),
+	 SDTC_BOOL(gui.scenario_developer,                   S,  0, false,                        STR_NULL,                                       NULL),
 	 SDTC_BOOL(gui.newgrf_show_old_versions,             S,  0, false,                        STR_NULL,                                       NULL),
 	  SDTC_VAR(gui.console_backlog_timeout,  SLE_UINT16, S,  0,   100,       10,    65500, 0, STR_NULL,                                       NULL),
 	  SDTC_VAR(gui.console_backlog_length,   SLE_UINT16, S,  0,   100,       10,    65500, 0, STR_NULL,                                       NULL),
diff --git a/src/toolbar_gui.cpp b/src/toolbar_gui.cpp
--- a/src/toolbar_gui.cpp
+++ b/src/toolbar_gui.cpp
@@ -319,7 +319,7 @@
 		case OME_GAMEOPTIONS:          ShowGameOptions();                               return;
 		case OME_DIFFICULTIES:         ShowGameDifficulty();                            return;
 		case OME_SETTINGS:             ShowGameSettings();                              return;
-		case OME_NEWGRFSETTINGS:       ShowNewGRFSettings(!_networking, true, true, &_grfconfig);   return;
+		case OME_NEWGRFSETTINGS:       ShowNewGRFSettings(!_networking && _settings_client.gui.scenario_developer, true, true, &_grfconfig);   return;
 		case OME_TRANSPARENCIES:       ShowTransparencyToolbar();                       break;
 
 		case OME_SHOW_TOWNNAMES:       ToggleBit(_display_opt, DO_SHOW_TOWN_NAMES);     break;