Hello, I’d like to know the community’s opinion on this type of migration.
To provide context, I’ve taken over the management of an old site under Sage 9. There’s a lot of content, and it really needs to be redone/migrated. Is a migration feasible, or is the issue not even relevant between two such different versions? Can the data migration be done simply if I simply create a new site with Sage 11?
I’ve migrated one site from Sage 9 → Sage 10 → Sage 11, and it was well worth it IMO.
For the Sage 9 → Sage 10 upgrade, I just created a new theme and moved everything over. The changes for getting the Javascript to work were the biggest and I expect it to be even easier if you go to Sage 11 directly since that way makes more sense to me. When I did the Sage 10 → Sage 11 update I didn’t create a new theme, instead I just did an in-place upgrade.
After the new 9 → 10 theme was done, I just deleted the old theme and moved the new one over, then I provisioned and deployed to staging to see if it works, and it did. You’ll have to set the page templates again though, aside from that everything was silk smooth.
I’d say go for it. We’re here if you get stuck. Focus on getting the templates working first so that the website actually loads, then move the CSS and then the JS.
How big of a site is it?
Thanks for your reply, it makes me want to get started! I’m wary because it’s an old site on which many devs have succeeded one another and added their own layers of complexity, libraries and plugins… I have custom plugins, post types in all directions and lots of features used wrongly and indiscriminately. The idea would therefore not be so much to migrate this site but to recover data to create a more recent version I think.
Don’t worry, the barely functioning spagetti code will continue being barely functioning spagetti code even with Sage 11, no more, no less. 
For what it’s worth, I’ve found that plugins work way better on Sage 10 or 11 than they ever did on Sage 9.
You can check out Acorn Post Types if you want to create the plugins in a simpler way.
If you’re willing to pay a little for a good starting point with Sage 11 you can invest in Radicle. It’s an official Roots project and it’s worth every penny IMO. It’s what we made our last client’s website in and it worked like a charm.
I’ll look into this, thanks for the recommendation! It didn’t take much to get me started, but I’m convinced.
First, I’m going to try to migrate all my data to a clean version of Sage 11 to start from a healthy base. If it’s too complex, I’ll try to migrate the existing code to Sage 11.
There’s a good chance I’ll come and reopen one or two topics in the coming weeks!