Despite the fact that the linked post is 10 years old:
I personally don't like SMF and never did like it, even when it was "new". I have managed many forums back in the days using Woltlab Burning Board 2/3 or vBulletin Board and always liked it way more, SMF felt to me like.... I don't realy know how to describe it, it felt... clunky and "cheap", like a cheap toy instead of a real tool.
But keep in mind that this is just a personal oppinion.
Also: Sure, you can argue, like Siwastaja did, that Software doesn't age if it just works. But keep in mind that there is more to something than a "well, it still works".
Besides the obvious safety related things - software bugs can be found even years after the development of something ceased and was thought as "perfect" or "done" - there is also the factor of useability and outward appearance, ease of administration, availability of addons, patches and updates and more I'm sure I forgot.
From those the outward appearance of SMF is one thing I personally don't like. It just looks cheap.
Problems while installing or managing addons seemed to have also happened in the past AFAIK?
I think I also read that there are some strange code workarounds you implemented which makes the management/administration a bit harder than it would need to be.
In my oppinion it would be a good thing to change, but in a sensible way:
Build a new software up, build import scripts and thoroughly check those, and let that run in the background for people to (explicitly and only) play with. When it works flawless set the old one to read only, import the data to the new software again and add a splash screen that tells the user about the change in software - that he can now only write in the new forums, which can be found *link*, and the old one, found *link*, is set to read only.
Then, after some time, remove that splash and redirect users directly to the new forums.
That should make up for a nice and easy transition in my experience.
But please, don't get one of those horrific softwares that don't have any visual structure anymore.