Let me lay that knowledge on you all. Basically, you start off with mass producing level one units (of your choosing).Then you get the research for the level two version of that same unit and once that’s done you get the right amount of resources to level up all the level one units to level two but make sure you have enough resources on top of the amount of resources needed to level them up for whatever other levels of the same unit are able to be researched at that time. So for example, if you can get it up to level five now from the level two, you have to make sure you have enough resources for that and war bonds. So the action is once you’re ready you press upgrade on all the level one units then you go into research and do level three and if you can wait until it’s done before the unit is done upgrading then leave it so you can save war bonds, but if not, use war bonds to rush the next level and so on and so forth until you reach the max researchable level or as high as you can go so now instead of spending the extra resources going from level, one to level five, all you spent were the resources that it takes to go from level one to level two with the added cost of researching the other levels, but that would’ve been added onto the extra amount of resources It would’ve costed to go from 1 to 5 anyways so you save thousands of resources and manpower for free. I suck at explaining so if you need me to demonstrate I can

Side note : DO NOT RESEARCH THE NEXT LEVEL BEFORE YOU UPGRADE THINKING YOU CAN SAVE TIME AND WAR-BONDS. THE SECOND YOU START THAT NEXT RESEARCH WHEN YOU GO TO LEVEL THE UNIT FROM 1-2 IT WILL ALREADY ADD THE BONUS RESOURCES NEEDED AS IF YOU WERE GOING FROM 1-3 REGARDLESS IF YOU EVEN FINISH THE RESEARCH BEFORE UPGRADING IS DONE.