Advice on rescheduling

One issue you'll be wanting to focus on is getting reliable information about average/probable weather from mid-October to early November. Mid-October to early November starts to get cold (for being outside) the more north you are, even northern Italy can get that way by then, plus when the weather turns colder there will be facilities/services that will start to be closing up for the winter. And the hours of daylight get shorter the later you go, a real factor that will reduce the amount of stuff you can see/do by an hour or two.

I have never been to Europe in November, or any time between November and early April. In 2015 I was in Croatia (mostly in Zabreb) from October 6 through October 12. Fall had arrived, starting at least a week earlier when I was in Bulgaria and Montenegro. It was beginning to rain fairly often, and one of my last days was unpleasantly chilly and just gushing rain. You can take a look at actual, day-by-day, historical weather statistics on timeanddate.com to see whether I was unlucky. The data goes back about ten years; I'd recommend checking the last five.

I think the situation in Italy might depend on what part of the country you'll be visiting.

Although weather is to some degree unpredictable, the reduction in daylight hours as you move from late June toward late December is a given.

We have been to Italy most months of the year. Hands down our favorite time to travel is October, preferably the first two weeks. It’s usually still nice and warm, at least during the days.

Not much to add except for the reduced daylight hours can be a pain. You’re at Canadian latitudes so those days start getting really short as you head into November.

I have been to Spain twice in late Oct/early November twice, beautiful weather in Madrid and Andalusia.

Obviously, the weather is going to be better in September. No one knows what’s going to happen with Covid.

Where were you planning on going in Croatia? The season on the islands and on the coast finishes at the end of September, at which time many places close up for the winter. Dubrovnik and Split are busy year round. Places in Zagreb will, obviously, be open.

Greatly depend where you are going and which kind of travel have you planned. If your idea is to visit cities, monuments and museums October/November is absolutely better than August: less crowded and much lower temperature (walk in Rome with 40°C is a nightmare!). On the opposite: if your target are beaches and sun is true the opposite. Even the area of Italy you are going affect the choice. For the Alps August is a wonderful period, while November is probably one of the worst (not yet covered with snow, but probably a lot of rainy days).

What do you need to reschedule?...other than flights? Do you really need to commit to a date in a few months, or can you afford to wait until late July? Alternatively, choose from one of the recommendations (sounds like early October), start in Croatia, and book lodgings on a free cancellation basis. Ensure you make note of what the last date is for free cancellation, as not all are 24 hours prior to check in.

I actually think that delaying it a few months will be a benefit. Depending on where you were going to be in Italy - August is an oven.

Since i work in a school, most of my travel is during the summer, but many years ago i was in Croatia from Sept through November. Personally, much of the appeal of Croatia is it’s beaches and coast. It does not have any popular must see sights, meaning no Leaning tower of Pisa, no Eiffel Tower, no Tower of London. Most of it’s beauty and appeal is Nature, such as Plitvice Lakes. I tell my friends who Hate museums and running around, who really like to relax on vacation, this is the country for them. That said, going too late in the year wouldn’t be my recommendation. Zagreb is a great city, but small and a few days would be sufficient.

The interior of Croatia can get quite chilly in October. The drop in temperatures between the coastal areas and the interior is significant.
If you go to Croatia then, stick to the coastal locations.
Italy enjoys the protection of the taller Alps, so October is still nice, albeit generally rainy toward the end of the month.