Melanie Hammond,
Here is an easy way to do this. In Web app builder under the basemap widget add a second basemap to one already in your basemap gallery like so.
Here I added the topo basemap to the streets one.
Now in the config_Basemap Gallery.json file find the location where both maps are added.
Add in the two line for "displayLevels": [], (in this example it is on lines 8 and 15). Now you can specify what levels you want the basemap to display and I will change automatically.
{
"title": "Streets",
"thumbnailUrl": "//provo.maps.arcgis.com/sharing/rest/content/items/d8855ee4d3d74413babfb0f41203b168/info/thumbnail/world_street_map.jpg",
"layers": [
{
"id": "World_Street_Map_8421",
"opacity": 1,
"displayLevels": [11,12,13,14,15,16,17,18],
"visibility": true,
"url": "http://services.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer"
},
{
"id": "topo",
"opacity": 1,
"displayLevels": [1,2,3,4,5,6,7,8,9,10],
"visibility": true,
"url": "http://services.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer"
}
],
"spatialReference": {
"wkid": "102100"
}
},
Hope this helps.
-Stan