There are two typical ways out of a specific scenario (or set of scenarios): the normal and expected way (even if it isn’t the most pleasing), and the tedious and overstimulating way.
With the benefit of having a quick and efficient leveling experience, a conversion feature would grant players the ability to choose between a slider and a numerical keypad to input the target progression status of their item. The game would then automatically convert those levels based on what the concurrent statistics are of the player in question, using whatever resource packs a player may have in store, or taking from the farms and mills as many times as necessary, with the caveat that reproduction of those resources lost would take a longer amount of time; the methods of resource gain and/or resource expenditure would be entirely based on the player’s desires.
In the worst-case scenario, the feature would be able to create its own automatic source of food or lumber to fulfill the task. When dealing with dragons, players would have to ensure they’ve accumulated enough XP prior to performing the task, or any XP potions could be used to do so, provided they’re at hand to begin with (XP could also be detracted from previously-experted dragons); when dealing with towers, all player XP would be accumulated promptly after the task has been performed.
This conversion option would abate some of the time that it takes to wait until you can get a tower or a dragon up to par.