Based on my experience it's a sizable amount of work. Nothing particularly big, but expect most of the time to be spent in testing.
Of course, each site is different, but the actual breaking changes are "managable", VS will catch most of them and you can fix them quite easily.
We will hopefully do a series regarding upgrading Foundation to .NET 5
Getting everything on .NET Standard version will help for the .NET 5/6 upgrade. It's worth nothing this isn't .NET Core anymore as the 5 version that the new preview is replaces any naming relating to .NET Core.
I'd suggest you get used to the .NET 5 preview and understand it. I've done a series of posts on some areas of it https://world.optimizely.com/blogs/scott-reed/dates/2021/7/-net-5-preview-breakdown--admin-area/ that might help. There's going to be a course at some points around the .NET 5 version so keep an eye out for that.
Also to be aware there's a migration tool which will help you migrate. Core areas to be aware of
If I was quoting this I'd probably put in around 8-10 days to upgrade (if experienced in Optimizely) and around 2-3 days of testing and another 2-3 days of bug fixing. As a rough point
we havent upgraded yet, as other work had got in the way. we did however, upgrade all our packages to the lastest non CMS 12 version as a starting point.
next step may well be updating all our libs to net standard and see where we go from there.
we have quite a large amount of custom code, partial routers segments etc, which afaik are no longer supported in 12, so think we will be doing a great deal more than 10 days worth of upgrading I would expect.
if you have managed to make any progress, would certainly be interested in what were any sticking points for you
We are using episerver.cms 11.14, a bit behind the curve - but are really wanting to migrate to net core as soon as it is available. I understand this currently to be Q4 this year.
I was wondering however if anyone knows how difficult migrating would be, how much code - or in which areas - is likely to need to be re-written.
None of our libraries are currently net standard, maybe this would be a good thing to try and start updating if that is at least moving in the right direction.