With the poster I was responding to, there well may not be much of a honeymoon period anymore. They are already familiar with an iPad, so the immediate fascination has already passed, and it's about getting right back to work with a device that you have already owned. For another friend who did change sizes, for them it was all about the focus on which size was preferable for their current lifestyle. They still had their original size, and had the luxury of being able to switch back & forth to see what was more comfortable for them. For them, that option was great in helping to make their ultimate decision.
For clarification: I am working with the presumption that the poster isn't selling their 12.9 off first and jumping to an 11. My guess is they would try the newer 11, then sell off the older 12.9 to offset cost, or give it to family/friend. With that in mind, for a time they would have two iPads, which they can make their decision with. 👍