The item id(guid) will not change. Saving that map or layer that was authored in map viewer classic and then you make changes or just click save in map viewer will not change the item id. Clicking Save As will give you a new item id but the old one will still exist. You should not see any differences reading in a map from map viewer classic into map viewer and you can test opening the map in map viewer any time and that will not change your map until you click save or save as.
If you are making changes in Map Viewer like switching to a vector symbol, using multiple label classes, multiple popup elements, group layers and other enhancements we have made to the web map will not be supported if its being read into an older application like Web AppBuilder and they will get downgraded to what the application supports. If you are using the Map Viewer Classic map in apps like Instant Apps, Experience Builder, Dashboards, StoryMaps and make changes in Map Viewer you wont see any downgrading because they all use the same SDK to read in maps and layers.
Hope this helps
