There is a few solutions you can do.
1) You can store an array of ints if you need the previous values for multiple entries.
2) I think an easier solution might be to have a boolean within the Screen.class like
. You can iterate through the loop and whatever
needs the upgrade will be automatically updated, then set that variable to false afterward.
3) If it needs to be updated to a certain point. You can store
in the Screen.class instead. This way you'll be able to tell a tower exactly how far you want it to upgrade. If the tower isn't at the number upgrade in the Screen.class, then it is time to upgrade it.