The party is deep into my 5e-updated classic Greyhawk Giants series.
The hill giants and their orc and goblin minions have attacked the Sterish city of Headwater and have taken about a quarter of the city. The party is about to embark on a mission to go behind enemy lines, kidnap and assassinate a stone giant who is critical to the hill giants' city / siege offensive as being their only trained engineer. While the party's patron recognizes that the stone giant needs to die, she also recognizes that at present the Stone Giant Thane has not joined the giant alliance and she wants to keep it that way. Thus, she is requesting that the party carry the body of the slain giant honorably back to his Thane in an effort to preserve a fragile peace between the humans and the stone giants. I would like the party to be able to accept this mission, without it becoming either a logistical challenge involving wagons and draft animals, or without loaning them a portable hole.
Instead, I would like to provide them with a version of the Enlarge / Reduce spell which is in all aspects identical to the original except that it can be upcast to extend the duration.
I figure the giant is 17 feet high and 1000 pounds; after reduce it would be 8.5 feet and a manageable 125 pounds.
For this version of Enlarge / Reduce I am proposing that:
When cast at 3rd level against living creatures, it lasts 10 minutes
When cast at 3rd level against objects, it lasts 1 hour
When cast at 4th level against living creatures, it lasts 1 hour (similar to polymorph)
When cast at 4th level against objects, it lasts 8 hours
When cast at 5th level against living creatures, it lasts 8 hours (much less than geas)
When cast at 5th level against objects, it lasts 24 hours
The party Wizard is currently 8th level and getting close to 9th. Requiring her to use both her fourth level slots and maintain concentration for the duration of travel every day in order to move swiftly and stealthily into the mountains with the body is just the right level of challenge for the group.
My only concern is that allowing this version of the spell to the party wizard permanently will have some unforeseen interaction that I will later regret with some other spell, ability, or item. This question, for example, asks about upcasting enlarge to permit two changes in creature size, and the answers identify the interaction with levitation and grappling being problematic. I am interested in a similar troubleshooting review.