I wouldn't upgrade with only 26 days to go, but that's because I don't want to have to look for a restaurant on the day, and I like to plan 4 to 6 months ahead
there might be some restaurants you'll be able to book on such short notice, but this will mostly be at odd times. So it might impact your plans.
Tinkerpeas' advice is wise. Have a look at restaurants, menus, and price the cost of your meals as if you were ordering, right now, in each restaurant.
if you're able to choose as many restaurants as you have nights onsite, and if you're ok with spending more time at restaurants, then you could upgrade.
But if you have a hard time choosing a restaurant, for each day, that fits into your plan (you'll need to know where you'll be on any given day) then don't upgrade and stick to QSDP
Trying to just walk up to the podium on the day is a solution. But if you're hungry and they tell you that you need to wait 60 minutes for a table, you'll probably fall back on a CS restaurant instead. And you face the risk of having unused TS credits at the end of your stay
also consider that TS meals will require a 18 to 20% tip on the pre-tax check. And you'll need to add this to the cost of the upgrade.
The question is not to know if, financially, it's worth it, because it is. But the question is to know if it fits your way of touring the parks. Some people just don't like being tied to a restaurant reservation, and some people couldn't do without those "breaks"