Most of the big concerns I’ve seen around have been brought up here already, so I will just add some personal thoughts
I feel like what you would want to achieve is a longer Ulduar, and shorter ToC.
This change might make Ulduar more relevant a bit longer, but doesn’t make ToC shorter at all. My worry with this is that if anything ToC will feel even less interesting, due to the less impactful gear.
Of course I understand not wanting to mess with arena seasons, but I also think that ilvl changes will not do much to change how long the ToC phase feels.
Class balance, and things like tank EHP especially, could be a big concern. It is hard to say exactly how this would play out with the information available however. There has been a lot of discussion on which parts of the game are impacted, but I also think that the size of this impact is important.
I think some more information on, or examples of, how gear pieces would look before and after this change could be useful.
Personally I do like the idea of making the gap between Ulduar and ToC gear smaller. However at the moment I find it hard to get fully on board due to how much uncertainty there is in the overall impact of these changes.
Perhaps with more information on how the changes would look and the impact on things like class balance and raid difficulty I’d be less skeptical.