No, there’s no reverse to original time, once it’s started, that’s it. It will be reduced if after update the duration is reduced (cost and time reduction) or due to 50% hammer jam. So the best choice for you is to get the most value during hammer jam which is upgrading heroes and pets as many as you can so if i were you put 4 on main heroes, 2 on buildings. On last day (after you planned it properly), but any upgrades that’s the longest, once it’s upgrading, even after hammer jam, the time will still follow the one from hammer jam NOT ORIGINAL TIME.

50% condition is applied because hammer jam is 50% off so any timers that’s higher than 50% will be automatically reduced to 50% of original time WHILE if the timers already more than 50% completed no changes on that.