I will top off barrels that have been sitting for a week 1/4-1/2 full. The trick is to mak it a point or a point and a half heavy, that way it won't ferment. Never had a problem. Learned it from Glen Goodrich. I will switch barrels if the grade changes too, then go back and top them off later when the grade changes back or next start up. I had some barrels of fancy sitting around the sugar house since 2018 that I bottled last spring and it was perfect. I never heard of reheating syrup in a canner to put into a drum until this summer my neighbor told me they did it this way. In 2004 we got our 4x12 and filter press we always went from our drawoff box to the drum. Our first drums were Dr. Pepper 15 gallon drums.